方法一

方法摘自http://blog.it985.com/9808.html
代码已改写为swift

class demo: UIViewController {
    fileprivate var navBarHairlineImageView: UIImageView?
    override func viewDidLoad() {
        // 找到导航条底部横线的view
        navBarHairlineImageView = self.findHairlineImageViewUnder(view: (self.navigationController?.navigationBar)!)
    }
    override func viewWillAppear(_ animated: Bool) {
        // 隐藏
        self.navBarHairlineImageView?.isHidden = true
    }
    override func viewWillDisappear(_ animated: Bool) {
        // 显示
        self.navBarHairlineImageView?.isHidden = false
    }
    /// 寻找导航条底部横线
    func findHairlineImageViewUnder(view: UIView) -> UIImageView? {
        if view.isKind(of: UIImageView.classForCoder()) && view.bounds.size.height <= 1 {
            return view as? UIImageView
        }
        for subView in view.subviews {
            let imageView = self.findHairlineImageViewUnder(view: subView)
            if imageView != nil {
                return imageView
            }
        }
        return nil
    }
}

方法二

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