Thanks Krendel for the answer.
For current version (I use 5.0.7), you need to use _post_content.
add_filter('acf/get_valid_field/type=wysiwyg', 'my_get_valid_field_post_content');
function my_get_valid_field_post_content( $field )
{
if ( $field['name'] == '_post_content' )
$field['type'] = 'textarea';
return $field;
}