您可以使用NSAttributedString做到这一点

例：

let underlineAttribute = [NSUnderlineStyleAttributeName: NSUnderlineStyle.StyleSingle.rawValue]
let underlineAttributedString = NSAttributedString(string: "StringWithUnderLine", attributes: underlineAttribute)
myLabel.attributedText = underlineAttributedString

编辑

为了使一个UILabel的所有文本具有相同的属性，建议您将UILabel子类化并覆盖文本，如下所示：

斯威夫特4.2

class UnderlinedLabel: UILabel {

override var text: String? {
    didSet {
        guard let text = text else { return }
        let textRange = NSMakeRange(0, text.count)
        let attributedText = NSMutableAttributedString(string: text)
        attributedText.addAttribute(NSAttributedString.Key.underlineStyle , value: NSUnderlineStyle.single.rawValue, range: textRange)
        // Add other attributes if needed
        self.attributedText = attributedText
        }
    }
}

斯威夫特3.0

class UnderlinedLabel: UILabel {

    override var text: String? {
        didSet {
            guard let text = text else { return }
            let textRange = NSMakeRange(0, text.characters.count)
            let attributedText = NSMutableAttributedString(string: text)
            attributedText.addAttribute(NSUnderlineStyleAttributeName , value: NSUnderlineStyle.styleSingle.rawValue, range: textRange)
            // Add other attributes if needed
            self.attributedText = attributedText
        }
    }
}

然后将您的文本像这样：

@IBOutlet weak var label: UnderlinedLabel!

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = "StringWithUnderLine"
    }

旧：

迅捷（2.0到2.3）：

class UnderlinedLabel: UILabel {

    override var text: String? {
        didSet {
            guard let text = text else { return }
            let textRange = NSMakeRange(0, text.characters.count)
            let attributedText = NSMutableAttributedString(string: text)
            attributedText.addAttribute(NSUnderlineStyleAttributeName, value:NSUnderlineStyle.StyleSingle.rawValue, range: textRange)
            // Add other attributes if needed

            self.attributedText = attributedText
        }
    }
}

Swift 1.2：

class UnderlinedLabel: UILabel {

    override var text: String! {
        didSet {
            let textRange = NSMakeRange(0, count(text))
            let attributedText = NSMutableAttributedString(string: text)
            attributedText.addAttribute(NSUnderlineStyleAttributeName, value:NSUnderlineStyle.StyleSingle.rawValue, range: textRange)
            // Add other attributes if needed

            self.attributedText = attributedText
        }
    }
}