class CaptchaAction extends \yii\captcha\CaptchaAction
{
    public function run()
    {
        $this->setHttpHeaders();
        \Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
        return $this->renderImage($this->getVerifyCode(true));
    }
}

public function actions()
    {
        return [
            'captcha' => [
                'class' => 'common\actions\CaptchaAction',
                'width' => 130,
                'height' => 48,
                'maxLength' => 5,
                'minLength' => 4,
                'offset'=>4
            ],
        ];
    }

<img onclick="changeCaptcha()" src="" width="130px" height="34px" class="login-captcha" alt="点击刷新验证码" id="captcha_img"/>
function changeCaptcha(){
      var url = urlcreate("<?=\yii\helpers\Url::toRoute(['captcha'])?>","_t="+Math.random());
      $("#captcha_img").attr('src',url)
}

class LoginForm extends Model
{
    public $username;
    public $password;
    public $captcha;
    public function rules()
    {
        return [
            [['username', 'password','captcha'], 'required'],
            ['captcha', 'captcha','captchaAction' => 'public/captcha'],
            ['password', 'validatePassword'],
            ['remember', 'safe'],
        ];
    }

 $this->createAction('captcha')->validate($verify_code,false);