参考自：https://www.cnblogs.com/He-Jing/p/3799394.html

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class FollowMouse : MonoBehaviour
{
    Vector3 world;
    public float moveSpeed = 0;

    void Update()
    {
        Vector3 targetPosition = Camera.main.WorldToScreenPoint(this.transform.position);
        Vector3 mousePosition = Input.mousePosition;    //鼠标的屏幕坐标
        if (Input.GetMouseButton(0))
        {
            mousePosition.z = targetPosition.z;     //在屏幕上鼠标Z坐标值为0，所以将物体的Z值赋给鼠标
            world.x = Camera.main.ScreenToWorldPoint(mousePosition).x;
            world.z = Camera.main.ScreenToWorldPoint(mousePosition).z;
            world.y = this.transform.position.y;
            //world.y = Camera.main.ScreenToWorldPoint(mousePosition).y;      //据说会产生穿透，我没试出来

            moveSpeed = 3;
        }
        //如果直接用==会存在精度差导致speed不变为0产生抖动
        if ((world - transform.position).magnitude <= 0.03f)
        {
            moveSpeed = 0;
        }
        this.transform.LookAt(world);
        this.transform.Translate(Vector3.forward * moveSpeed * Time.deltaTime);
    }
}