Problem Solved:

The main problem is that my user table id is customized which is not a auto_increment id.

After adding public $incrementing = false; to my user model. The problem solved.

class User extends Model implements AuthenticatableContract, CanResetPasswordContract
{
    use Authenticatable, CanResetPassword;


    public $incrementing = false;

    /**
     * The database table used by the model.
     *
     * @var string
     */
    protected $table = 'register_info';

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = ['user_id','username', 'email', 'password','city'];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = ['password', 'remember_token'];

    /**
     * The primary key for the model.
     *
     * @var string
     */
    protected $primaryKey = 'user_id';

    /**
     * Get the bonding UserDetail
     *
     * @return UserDetail
     */
    public function getUserDetail()
    {
        return $this->hasOne('App\Model\UserDetail','user_id','user_id');
    }

}

Problem analysis: In laravel, by default the id of model user is set as auto-increment. If we use customized user_id like i do.

 /**
     * Create a new user instance after a valid registration.
     *
     * @param  array  $data
     * @return User
     */
    protected function create(array $data)
    {

        return User::create([
            'user_id' => uniqid('us'),
            'username' => $data['username'],
            'email' => $data['email'],
            'password' => bcrypt($data['password'])
        ]);
    }

The returned model will contain a auto-increment id. What i got is always 0. Therefore, a User instance with id 0 is stored in laravel. When i call Auth:user(), we will get null instead of the User instance.

{"user_id":"0","username":"[email protected]","password":"xxxxxxx"}

After adding public $incrementing = false; to my user model. I can get my user instance correctly with id that i defined.

{"user_id":"us55f7e7afbe87f","username":"[email protected]","password":"xxxxxxx"}

Hope this can be helpful.

Thanks everyone helped before.