在viewWillAppear 方法中敲以下代碼：

- (void)viewWillAippear:(BOOL)animated {
    [super viewWillAppear:animated];

    //去除導(dǎo)航欄下方的橫線
    [self.navigationController.navigationBar setBackgroundImage:[[UIImage alloc] init]
        forBarMetrics:UIBarMetricsDefault];
    [self.navigationController.navigationBar setShadowImage:[[UIImage alloc] init]];
}