using System.IO.Ports;

首先，如果你在此处已经出现问题，那就到Build Settings，把 .NET Standard 2.0改成 .NET 4.x
（没有这个选项的话就是 .Net.2.0 Subset改为 .Net.2.0）

public class testScript : MonoBehaviour
{
    
    
    public Text debug_text;

    private SerialPort sp;

    private void Start()
    {
    
    
        sp = new SerialPort("COM3", 9600);	
        //COM3 在我的电脑》属性》设备管理器查看
        //9600 是波特率，参见输出设备
        sp.ReadTimeout = 500;
        sp.Open();
    }
    
    string strRec;
    private void Update()
    {
    
    
        if (this.sp != null && this.sp.IsOpen)
        {
    
    
            try
            {
    
    
                strRec = sp.ReadLine();

            }
            catch
            {
    
    
                strRec = "not found";
            }
            debug_text.text = "Receive From Serial: " + strRec;

        }
        else
        {
    
    
            debug_text.text = "null SP or closed. ";
        }
    }