例：

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

编辑

为了对一个UILabel的所有文本具有相同的属性,我建议您将UILabel子类化并覆盖文本,如下所示：

Swift 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
        }
    }
}

你把你的文本这样：

@IBOutlet weak var label: UnderlinedLabel!

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = "StringWithUnderLine"
    }

旧：

Swift(2.0至2.3)：

class UnderlinedLabel: UILabel {

    override var text: String? {
        didSet {
            guard let text = text else { return }
            let textRange = NSMakeRange(0,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,range: textRange)
            // Add other attributes if needed

            self.attributedText = attributedText
        }
    }
}