效果图：

一、场景布局

1.制作坐标轴

 设置图层

2.相机设置

效果：设置坐标模型不被其他物体遮挡

新建空节点作为相机父节点，创建一个新相机（注：空节点下有两个相机 一个Main Camera主相机  一个显示坐标模型相机）如图：

给相机父物体GameObjec添加脚本，用于旋转相机 观察操作Cube

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

public class CameraController : MonoBehaviour
{
    public Transform cameraParent;

    private const float MOVE_SPEED = 0.1F;
    private const float ROTATE_SPEED = 0.1F;

    private Vector3 lastPos = Vector3.zero;

    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        if (Input.GetMouseButtonDown(1))
        {
            lastPos = Input.mousePosition;
        }
        if (Input.GetMouseButton(1))//鼠标右键按下 旋转摄影机
        {
            RotateCamera();
        }
        MoveCamera();
    }

    private void MoveCamera()
    {
        float mouse = Input.GetAxis("Mouse ScrollWheel");
        Vector3 pos = cameraParent.Find("Camera").position;
        if (mouse < 0)
        {
            pos.z -= MOVE_SPEED;
        }
        if (mouse > 0)
        {
            pos.z += MOVE_SPEED;
        }
        cameraParent.Find("Main Camera").position = pos;
        cameraParent.Find("Camera").position = pos;
    }

    private void RotateCamera()
    {
        float dx = Input.mousePosition.x - lastPos.x;
        float dy = Input.mousePosition.y - lastPos.y;

        Vector3 rotation = cameraParent.rotation.eulerAngles;
        rotation.x -= dy * ROTATE_SPEED;
        rotation.y += dx * ROTATE_SPEED;
        cameraParent.rotation = Quaternion.Euler(rotation);

        lastPos = Input.mousePosition;
    }
}

 3.坐标轴脚本

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

public class MoveTargetItem : MonoBehaviour
{
    public Transform axis;  //坐标轴模型
    public Transform cube;  //要移动的物体
    public Camera axisCamera; //只渲染坐标轴的摄像机

    public float MOVE_SPEED = 0.03F;

    private int currentAxis = 0;//当前要移动的轴 用 1 2 3标识 x y z轴 0表示没有选择坐标轴
    private bool choosedAxis = false;//判断是否选择了坐标轴
    private Vector3 oldPos; //上一帧鼠标位置

    void Start()
    {
        axis.position = cube.position;
    }

    void Update()
    {
        //鼠标按下
        if (Input.GetMouseButtonDown(0))
        {
            //检测相机是否碰撞了坐标轴
            Ray ray = axisCamera.GetComponent<Camera>().ScreenPointToRay(Input.mousePosition);
            RaycastHit hit;
            if (Physics.Raycast(ray, out hit, 1000, 1 << LayerMask.NameToLayer("Axis"))) //只检测Axis这一层
            {
                choosedAxis = true;
                //碰撞成功后 记录鼠标位置
                oldPos = Input.mousePosition;

                //判断当前选择的坐标轴
                switch (hit.collider.name)
                {
                    case "x":
                        currentAxis = 1;
                        break;
                    case "y":
                        currentAxis = 2;
                        break;
                    case "z":
                        currentAxis = 3;
                        break;
                    default:
                        break;
                }
            }
        }

        //鼠标长按并且在按下鼠标时选中坐标轴
        if (Input.GetMouseButton(0) && choosedAxis)
        {
            //移动物体
            UpdateCubePosition();
        }

        //鼠标抬起后初始化数值 
        if (Input.GetMouseButtonUp(0))
        {
            choosedAxis = false;
            currentAxis = 0;
        }
    }

    //开始移动物体
    private void UpdateCubePosition()
    {
        Vector3 origin = axisCamera.WorldToScreenPoint(axis.position);  //坐标轴原点对应屏幕坐标
        Vector3 mouseMovePos = Input.mousePosition - oldPos;   

        //三个坐标轴的终点对应屏幕坐标
        Vector3 axisEnd_x = axisCamera.WorldToScreenPoint(axis.Find("x/x").position);
        Vector3 axisEnd_y = axisCamera.WorldToScreenPoint(axis.Find("y/y").position);
        Vector3 axisEnd_z = axisCamera.WorldToScreenPoint(axis.Find("z/z").position);

        //x、y、z轴与原点在屏幕坐标上的差值
        Vector3 vector_x = axisEnd_x - origin;
        Vector3 vector_y = axisEnd_y - origin;
        Vector3 vector_z = axisEnd_z - origin;

        //获取移动物体的世界坐标
        Vector3 cubePos = cube.position;
        float d = Vector3.Distance(Input.mousePosition, oldPos) * MOVE_SPEED; //鼠标拖动距离
        float cos;//获取
        switch (currentAxis)
        {
            case 1:

                /*鼠标移动轨迹与X轴夹角的余弦值
                 *  Vector3.Angle(V3,V3) 计算两个向量之间的夹角
                 *  派（3.141592）/180 等于一角度等于多少弧度
                 *  Mathf.Cos(弧度值)返回以弧度表示的角f的余弦值。
                 */
                cos = Mathf.Cos(Mathf.PI / 180 * Vector3.Angle(mouseMovePos, vector_x));

                //根据余弦值的大小 判断移动物体的前进后退
                if (cos < 0) { d = -d; }
                cubePos.x += d;
                break;

            case 2:
                cos = Mathf.Cos(Mathf.PI / 180 * Vector3.Angle(mouseMovePos, vector_y));
                if (cos < 0) { d = -d; }
                cubePos.y += d;
                break;
            case 3:
                cos = Mathf.Cos(Mathf.PI / 180 * Vector3.Angle(mouseMovePos, vector_z));
                if (cos < 0) { d = -d; }
                cubePos.z += d;
                break;

            default:
                break;
        }

        cube.position = cubePos;
        axis.position = cubePos;
        oldPos = Input.mousePosition;
    }
}