试了好久，终于成功了！

客户端代码如下：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System.Net;
using System.Net.Sockets;
using System.Text;
public class TestNet : MonoBehaviour {

	// Use this for initialization
	void Start () {
        print("connect");
        System.Net.Sockets.TcpClient client = new TcpClient();
        client.Connect(new IPEndPoint(IPAddress.Parse("127.0.0.1"), 8080));
        if (client.Connected)
        {
            //while (true)
            {
                var stream = client.GetStream();
                byte[] buffer = new byte[1024];
                buffer = Encoding.UTF8.GetBytes("123");
                stream.Write(buffer, 0, buffer.Length);

                byte[] buffer2 = new byte[1024];
                stream.Read(buffer2, 0, buffer2.Length);
                
                Debug.Log(Encoding.UTF8.GetString(buffer2,0, buffer2.Length));

            }
            client.Close();
        }
    }
	
	// Update is called once per frame
	void Update () {
		
	}
}

服务端是用python写的，见我的另一篇博文：
https://blog.csdn.net/u013749051/article/details/85455517