1.第一种方法
[[UINavigationBar appearance] setBackgroundImage:[[UIImage alloc] init] forBarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault];
[[UINavigationBar appearance] setShadowImage:[[UIImage alloc] init]];
2.第二种方法
@interface BaseViewController (){
UIImageView *navBarHairlineImageView;
}

• (void)viewDidLoad {
  [super viewDidLoad];
  navBarHairlineImageView = [self findHairlineImageViewUnder:self.navigationController.navigationBar];

  // Do any additional setup after loading the view from its nib.
  }

• (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;
  }

• (void)viewWillAppear:(BOOL)animated
  {
  [super viewWillAppear:animated];
  navBarHairlineImageView.hidden = YES;
  }

• (void)viewWillDisappear:(BOOL)animated
  {
  [super viewWillDisappear:animated];
  navBarHairlineImageView.hidden = NO;
  }