第一种方法。（可以保证导航条原来的透明）

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    self.navigationController.navigationBar.barTintColor = [UIColor whiteColor];
    if ([self.navigationController.navigationBar respondsToSelector:@selector( setBackgroundImage:forBarMetrics:)]){
        NSArray *list=self.navigationController.navigationBar.subviews;
        for (id obj in list) {
            if ([obj isKindOfClass:[UIImageView class]]) {
                UIImageView *imageView=(UIImageView *)obj;
                NSArray *list2=imageView.subviews;
                for (id obj2 in list2) {
                    if ([obj2 isKindOfClass:[UIImageView class]]) {
                        UIImageView *imageView2=(UIImageView *)obj2;
                        imageView2.hidden=YES;
                    }
                }
            }
        }
    }
}

第二种方式。（这种方式相对简单一些，但是这种效果会去掉导航条的透明效果）

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

第三种方式。（比较常用，也比较实用）

//隐藏分割线
-(void)viewWillAppear:(BOOL)animated{
    [super viewWillAppear:animated];
    self.navigationController.navigationBar.barTintColor = [UIColor whiteColor];
    self.navigationImageView = [self findHairlineImageViewUnder:self.navigationController.navigationBar];
    self.navigationImageView.hidden = YES;

}
//显示分割线
-(void)viewDidDisappear:(BOOL)animated{
    [super viewDidDisappear:animated];
    self.navigationImageView.hidden = NO;
}
//实现的方法
-(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;
}