你的例子 Alamofire.request(.POST, "http://mywebsite.com/post-request", parameters: ["foo":"bar"]) 已经包含 "foo=bar" 字符串作为它的主体.但是如果你真的想要自定义格式的字符串.你可以这样做:

Your example Alamofire.request(.POST, "http://mywebsite.com/post-request", parameters: ["foo": "bar"]) already contains "foo=bar" string as its body. But if you really want string with custom format. You can do this:

Alamofire.request(.POST, "http://mywebsite.com/post-request", parameters: [:], encoding: .Custom({
            (convertible, params) in
            var mutableRequest = convertible.URLRequest.copy() as NSMutableURLRequest
            mutableRequest.HTTPBody = "myBodyString".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)
            return (mutableRequest, nil)
        }))

注意:参数不能为nil

更新(Alamofire 4.0、Swift 3.0):

在 Alamofire 4.0 中 API 已更改.因此对于自定义编码，我们需要符合 ParameterEncoding 协议的值/对象.

In Alamofire 4.0 API has changed. So for custom encoding we need value/object which conforms to ParameterEncoding protocol.

extension String: ParameterEncoding {

    public func encode(_ urlRequest: URLRequestConvertible, with parameters: Parameters?) throws -> URLRequest {
        var request = try urlRequest.asURLRequest()
        request.httpBody = data(using: .utf8, allowLossyConversion: false)
        return request
    }

}

Alamofire.request("http://mywebsite.com/post-request", method: .post, parameters: [:], encoding: "myBody", headers: [:])