I want to load post content by categories using Ajax

Here, i have develop a simple demo for get a post data of depends on category without loading page using Ajax and jQuery.

Here is a main category, sub category and post dropdowns :

$args = array(
        'orderby' => 'name',
        'hierarchical' => 1,
        'taxonomy' => 'category',
        'hide_empty' => 1,
        'parent' => 0,
        );
        $categories = get_categories($args);
        ?>
        <select id="main_cat">
            <option value="0">-----Select Category------</option>
        <?php
        foreach($categories as $category) {
            $option .= '<option value="'.$category->cat_ID.'">';
                 $option .= $category->name;
                $option .= '</option>';

        } 
        echo $option;
        ?>
                    </select>
    <select id="sub_cat">
    <option value="0">-----Select Sub Cat------</option> 
    </select>
    <select id="get_this_post">
    <option value="0">------Select Post-----</option> 
    </select>
    <div id="post-content-wrapper">
    </div> 

Now jQuery part: get a change events of dropdowns and retrieve data using ajax action.

frontend_ajax_object is a object of ajax url. you have to change according as you defined.

<script type="text/javascript">
jQuery(function($){
            $('#main_cat').change(function(){
                    var $mainCat=$('#main_cat').val();

                    // call ajax
                    console.log(frontend_ajax_object.ajaxurl);
                     $("#sub_cat").empty();
                     $("#get_this_post").empty();
                      $("#post-content-wrapper").html("");
                        $.ajax({
                            url:frontend_ajax_object.ajaxurl,
                            type:'POST',
                             data:'action=get_sub_category&main_catid=' + $mainCat,
                             success:function(results)
                             {
                                //  alert(results);
                                    $("#sub_cat").removeAttr("style");
                                    $("#sub_cat").append(results);
                             }
                        });
                     }
             );

             $('#sub_cat').change(function(){
                    var $sub_catid=$('#sub_cat').val();

                    // call ajax
                     $("#get_this_post").empty();
                     $("#post-content-wrapper").html("");
                        $.ajax({
                            url:frontend_ajax_object.ajaxurl,
                            type:'POST',
                             data:'action=get_sub_category_posts&sub_catid='+ $sub_catid,
                             success:function(results)
                             {
                               //  alert(results);
                                // $("#sub_cat").removeAttr("style");
                                  $("#get_this_post").append(results);
                             }
                        });
                     }
             );

             $('#get_this_post').change(function(){
                    var $get_this_post=$('#get_this_post').val();

                    // call ajax
                     $("#post-content-wrapper").html("");
                        $.ajax({
                            url:frontend_ajax_object.ajaxurl,
                            type:'POST',
                             data:'action=get_page_data&this_post_id='+ $get_this_post,
                             success:function(results)
                             {
                               //  alert(results);
                                $("#post-content-wrapper").html(results);
                             }
                        });
                     }
             );
});
</script>

Now, here is a ajax function which is used to get a data based on selected fields.

add_action('wp_ajax_get_sub_category', 'get_sub_category');
add_action('wp_ajax_nopriv_get_sub_category', 'get_sub_category');
function get_sub_category() {
    if(isset($_POST['main_catid']))
    {
        global $post;

        if(!empty($_POST['main_catid']))
        {
            $args = array('parent' => $_POST['main_catid']);
            $categories = get_categories( $args );
            $option = "<option value="0">Select sub cat</option>";
            foreach($categories as $category) { 
                $option .= '<option value="'.$category->term_id.'">';
                 $option .= $category->name;
                $option .= '</option>';

            }
            echo $option;
            wp_die();
        } // end if
    }
}

add_action('wp_ajax_get_sub_category_posts', 'get_sub_category_posts');
add_action('wp_ajax_nopriv_get_sub_category_posts', 'get_sub_category_posts');
function get_sub_category_posts() {
    if(isset($_POST['sub_catid']))
    {
        global $post;

        if(!empty($_POST['sub_catid']))
        {
            $args = array( 'posts_per_page' => 5, 'offset'=> 0, 'category' => $_POST['sub_catid'] );

            $myposts = get_posts( $args );
            $option = "<option value="0">Select Post</option>";
            foreach( $myposts as $post ) : setup_postdata($post); 
                 $option .= '<option value="'.get_the_ID().'">';
                 $option .= get_the_title();
                $option .= '</option>';
            endforeach; 
            echo $option;
            wp_die();
        } // end if
    }
}
add_action('wp_ajax_get_page_data', 'get_page_data');
add_action('wp_ajax_nopriv_get_page_data', 'get_page_data');
function get_page_data() {
    if(isset($_POST['this_post_id']))
    {
        global $post;

        if(!empty($_POST['this_post_id']))
        {
            $p_id= $_POST['this_post_id'];

            echo '<h1>'.get_the_title($p_id).'</h1>';
            echo '<p class="the_content">'.get_post_field('post_content', $p_id).'</p>';

            wp_die();
        } // end if
    }
}

I hope this demo will helps you. i have tested and it is working fine. let me know if this is helps you!

Thank you!