public $verifyCode;
public function rules()
{
   return [
      ……
      ['verifyCode', 'captcha', 'on' => 'login'], //验证码
   ];
}
public function scenarios() {
    $scenarios = parent::scenarios();
    $scenarios['login'] = ['username', 'password', 'rememberMe', 'verifyCode'];
    return $scenarios;
}

public function actions()
{
    return [
      'error' => [
        'class' => 'yii\web\ErrorAction',
      ],
      'captcha' => [
        'class' => 'yii\captcha\CaptchaAction',
        'fixedVerifyCode' => YII_ENV_TEST ? 'testme' : null,
        'width' => 100,
        'height' => 34,
        'padding' => 0,
        'minLength' => 4,
        'maxLength' => 4,
      ],
    ];
}
public function actionLogin()
{
    if (!\Yii::$app->user->isGuest) {
      return $this->goHome();
    }
    $model = new LoginForm(['scenario' => 'login']);
    if ($model->load(Yii::$app->request->post()) && $model->login()) {
      return $this->goBack();
    } else {
      return $this->render('login', [
        'model' => $model,
      ]);
    }
}

<?php
use yii\helpers\Html;
use yii\bootstrap\ActiveForm;
use yii\captcha\Captcha;
?>
<?php $form = ActiveForm::begin(['id' => 'login-form']); ?>
<?= $form->field($model, 'verifyCode')->widget(Captcha::className(), [
          'template' => '<div class="input-group input-group-lg col-md-8"><span class="input-group-addon"><i class="glyphicon glyphicon-eye-open red"></i></span>{input}<div class="input-group-addon" style="padding:5px;">{image}</div></div>',
          'options' => ['class' => 'form-control','placeholder'=>"验证码"],
          'imageOptions'=>['alt'=>'点击换图','title'=>'点击换图', 'style'=>'cursor:pointer']
 ])->label(false) ?>
<?php ActiveForm::end(); ?>