get all sub categories without specify any category

You could use get_categories().

Returns an array of category objects matching the query parameters.

Arguments are pretty much the same as wp_list_categories and can be passed as either array or in query syntax.

// Fetch parent categories
$parent_categories = get_categories( 'parent=0' );

foreach ( $parent_categories as $parent_category ) {
  // Fetch child categories
  $args = array(
    'parent' => $parent_category->term_id
  );

  $categories = get_categories( $args );

  foreach ( $categories as $category ) {
    printf( '<div>%s</div>', $category->name );
  }
}

This is a very simple example of the code without extra parameters like hide_empty, type etc.