第一种：

先设置一个全局变量 UIImageView * imageView;

然后：

- (void)viewWillAppear:(BOOL)animated{    [super viewWillAppear:animated];    _imageView.hidden = YES;}- (void)viewWillDisappear:(BOOL)animated{    [super viewWillDisappear:animated];    _imageView.hidden = NO;}- (void)viewDidLoad {    [super viewDidLoad];        _imageView = [self findHairlineImageViewUnder:self.navigationController.navigationBar];}- (UIImageView *)findHairlineImageViewUnder:(UIView *)view {    if ([view isKindOfClass:UIImageView.class] && view.bounds.size.height <= 1.0) {        return (UIImageView *)view;    }    for (UIView *subview in view.subviews) {        UIImageView *imageView = [self findHairlineImageViewUnder:subview];        if (imageView) {            return imageView;        }    }    return nil;}

第二种：

直接在AppDelegate.m中设置，可全局生效

[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc] init] forBarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault];    [[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]];