Python Sever:

import threading
from socket import *

HOST = ''
PORT = 21567
BUFSIZ = 1024
ADDR = (HOST, PORT)

tcpSerSock = socket(AF_INET, SOCK_STREAM)
tcpSerSock.bind(ADDR)
tcpSerSock.listen(8)

def thread_it(func, *args):
    '''将函数打包进线程'''
    t = threading.Thread(target=func, args=args)
    t.setDaemon(False)
    t.start()

def acceptSuc(tcpCliSock, addr):
    print('有一个客户端连接成功')
    while True:
        try:
            recv_data = tcpCliSock.recv(BUFSIZ)
        except ConnectionResetError:
            print('有一个客户端关闭了连接')
        if recv_data:
            data=recv_data.decode('utf-8')
            print('接收到%s' % data)
            tcpCliSock.send((data+" received").encode())

while True:
    print('waiting for connection...')
    tcpCliSock, addr = tcpSerSock.accept()
    print('...connnecting from:', addr)
    thread_it(acceptSuc, tcpCliSock, addr)
    print(tcpCliSock)

Unity Client:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
public class TcpClient : MonoBehaviour
{
    public static IPAddress ip;
    IPEndPoint ipEnd;
    Thread connectThread;
    public static Socket serverSocket;
    string recvStr; //接收的字符串
    string sendStr; //发送的字符串
    public static  byte[] sendData=new byte[1024];//接收的数据，必须为字节
    byte[] recvData=new byte[1024];//发送的数据，必须为字节
    int recvLen; //接收的数据长度

    string editString="";

    void InitSocket()
    {
        // ip=IPAddress.Parse("x.x.x.x");
        ip=IPAddress.Parse("127.0.0.1");

        ipEnd=new IPEndPoint(ip,21567);
        connectThread=new Thread(new ThreadStart(SocketReceive));
        connectThread.Start();
        
    }

    void SocketConnet()
    {
        if(serverSocket!=null)
            serverSocket.Close();
        serverSocket=new Socket(AddressFamily.InterNetwork,SocketType.Stream,ProtocolType.Tcp);
        Debug.Log("Ready to connect.");

        serverSocket.Connect(ipEnd);  

    }


    static string? UTF8String( string? input)
    {
        UTF8Encoding utf8 =? new? UTF8Encoding();
        return? utf8.GetString(utf8.GetBytes(input));
    }

    public static void SocketSend(string sendStr)
    {
        Debug.Log(sendStr);
        TcpClient.sendData=new byte[1024];
        TcpClient.sendData=Encoding.ASCII.GetBytes(sendStr);
        serverSocket.Send(TcpClient.sendData,TcpClient.sendData.Length,SocketFlags.None);
    }

    void SocketReceive()
    {
        SocketConnet();
        while(true)
        {
            recvData=new byte[1024];
            recvLen=serverSocket.Receive(recvData);
            if(recvLen==0)
            {
                SocketConnet();
                continue;
            }
????????    UTF8Encoding utf8 =? new? UTF8Encoding();
            recvStr=utf8.GetString(recvData,0,recvLen);
            Debug.Log(recvStr);
        }        
    }

    void SocketQuit()
    {
        //关闭线程
        if(connectThread!=null)
        {
            connectThread.Interrupt();
            connectThread.Abort();
        }
        //最后关闭服务器
        if(serverSocket!=null)
            serverSocket.Close();
        print("diconnect");
    }

    void Start()
    {
        InitSocket();
    }

    void OnApplicationQuit()
    {
        SocketQuit();
    }

    void OnGUI()
    {
        int xpos=10;
        editString=GUI.TextField(new Rect(xpos,10,500,20),editString);
        string split=((char)1).ToString();
        if(GUI.Button(new Rect(xpos,30,60,20),"send"))
            SocketSend(editString);
    }

}