OK, I find that the image class which is already used 100% as the quality.
Then, I really need to improve the quality of the source image file.
Thanks!
public function save($file, $quality = 100) {
$extension = pathinfo($file,PATHINFO_EXTENSION);
if ($extension == 'jpeg' || $extension == 'jpg') {
imagejpeg($this->image, $file, $quality);