直接挂载到模型上

using UnityEngine;
using System.Collections;

public class OnDrag : MonoBehaviour
{

    //目标物体
    public Transform target;
    private int MouseWheelSensitivity = 1; //放大倍数的快慢
    private int MouseZoomMin = 1; //最小倍数
    private int MouseZoomMax = 5; //最大倍数
    //默认距离
    private float normalDistance = 3;


    private Vector3 normalized;

    //拖拽的移动速度
    private float xSpeed = 250.0f;
    private float ySpeed = 120.0f;

    //拖拽的高度限制
    private int yMinLimit = -20;
    private int yMaxLimit = 80;

    //角度
    private float x = 0.0f;
    private float y = 0.0f;

    //记录目标物体的坐标
    private Vector3 screenPoint;
    private Vector3 offset;

    //围绕x旋转30°
    private Quaternion rotation = Quaternion.Euler(new Vector3(30f, 0f, 0f));

    //目标的3D坐标
    private Vector3 CameraTarget;

    //打印欧拉角:绕各个轴旋转的角度，顺时针为正方向
    public void Awake()
    {
        target = transform;
        print(transform.eulerAngles.x);
        print(transform.eulerAngles.y);
        print(transform.eulerAngles.z);
    }

    void Start()
    {
        //找到目标飞机的3d坐标
        CameraTarget = target.position;

        //目标飞机的z-3，距离摄像机更近了
        float z = target.transform.position.z - normalDistance;
        //给当前相机给定位，现在的3D坐标乘以30°
        transform.position = rotation * new Vector3(transform.position.x, transform.position.y, z);

        //将视角转向物体
        transform.LookAt(target);

        //记录各个轴偏离的角度
        var angles = transform.eulerAngles;
        x = angles.y;
        y = angles.x;
    }

    void OnGUI()
    {
        GUI.Label(new Rect(10, 10, 300, 30), "左击:旋转；滚轮：缩放；中击：拖拽");
    }

    void Update()
    {
        //如果左击了，旋转
        if (Input.GetMouseButton(0))
        {
            x += Input.GetAxis("Mouse X") * xSpeed * 0.02f;
            y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;

            y = ClampAngle(y, yMinLimit, yMaxLimit);

            var rotation = Quaternion.Euler(y, x, 0);
            var position = rotation * new Vector3(0.0f, 0.0f, -normalDistance) + CameraTarget;

            transform.rotation = rotation;
            transform.position = position;

        }
        //滚轮缩放
        else if (Input.GetAxis("Mouse ScrollWheel") != 0)
        {
            //摄像机3d坐标-物体的3d坐标
            normalized = (transform.position - CameraTarget).normalized;

            if (normalDistance >= MouseZoomMin && normalDistance <= MouseZoomMax)
            {
                normalDistance -= Input.GetAxis("Mouse ScrollWheel") * MouseWheelSensitivity;
            }
            if (normalDistance < MouseZoomMin)
            {
                normalDistance = MouseZoomMin;
            }
            if (normalDistance > MouseZoomMax)
            {
                normalDistance = MouseZoomMax;
            }
            //改变摄像机的远近
            transform.position = normalized * normalDistance;
        }
        //案件按下 记录鼠标的
        else if (Input.GetMouseButtonDown(2))
        {
            //将目标物体的坐标转化成平面坐标
            screenPoint = Camera.main.WorldToScreenPoint(target.transform.position);
            //计算鼠标的3维坐标跟物体的3维坐标的差值
            offset = target.transform.position - Camera.main.ScreenToWorldPoint(new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z));
        }

        //中键拖拽，改变飞机的坐标，每帧调用
        if (Input.GetMouseButton(2))
        {
            //鼠标的平面坐标
            Vector3 curScreenPoint = new Vector3(Input.mousePosition.x, Input.mousePosition.y, screenPoint.z);
            //鼠标转移的3d空间坐标值
            Vector3 curPosition = Camera.main.ScreenToWorldPoint(curScreenPoint) + offset;
            //改变鼠标的3D空间坐标
            target.transform.position = curPosition;
        }
        //朝向，每动一帧都要改变朝向
        transform.LookAt(CameraTarget);
    }

    //控制旋转的角度，如果旋转的角度大于360或者小于360都要加上或者减去对应的角度
    static float ClampAngle(float angle, float min, float max)
    {
        if (angle < -360)
            angle += 360;
        if (angle > 360)
            angle -= 360;
        return Mathf.Clamp(angle, min, max);
    }
}