试试这个

let base64Encoded = "YW55IGNhcm5hbCBwbGVhc3VyZS4="

let decodedData = Data(base64Encoded: base64Encoded)!
let decodedString = String(data: decodedData, encoding: .utf8)!

print(decodedString)

println(decodedString)

确保您的 base 64 编码字符串有效

make sure your base 64 encoded string is valid