效果如下：

代码如下：

public class TPSCamera : MonoBehaviour
{
    /// <summary>
    /// 目标对象
    /// </summary>
    [SerializeField]
    Transform target = null;

    /// <summary>
    /// 旋转参数
    /// </summary>
    [SerializeField]
    Vector2 rotate;

    /// <summary>
    /// 旋转速度
    /// </summary>
    [SerializeField]
    float rotateSpeed = 2;

    /// <summary>
    /// 视口大小
    /// </summary>
    [SerializeField]
    float viewSize = 30;

    /// <summary>
    /// 默认角度
    /// 在目标对象的哪个方向
    /// </summary>
    [SerializeField]
    float defaultAngle = -135;

    /// <summary>
    /// 离目标对象的距离
    /// </summary>
    [SerializeField]
    float radius = 3;

    /// <summary>
    /// 离目标对象的高度
    /// </summary>
    [SerializeField]
    float height = 1.5f;

        public bool Aim;

    public bool visiable = false;

    public CursorLockMode lockMode;

    Camera cam;

    /// <summary>
    /// 绕任意轴旋转的参考点
    /// 暂时用这个办法替代
    /// </summary>
    Transform tr;

    static TPSCamera _inst;

    public static TPSCamera inst
    {
        get {
            return _inst;
        }
    }

    void Awake ()
    {
        _inst = this;
    }

    void Start ()
    {
        cam = this.GetComponent<Camera> ();
        tr = new GameObject ().transform;
    }

    void FixedUpdate ()
    {
        rotate.x += Input.GetAxis ("Mouse X") * rotateSpeed;
        rotate.y += Input.GetAxis ("Mouse Y") * rotateSpeed;
        viewSize -= Input.mouseScrollDelta.y * 3;

        //一些约束，不用管
        if (viewSize < 30)
        {
            viewSize = 30;
        } else if (viewSize > 60)
        {
            viewSize = 60;
        }

        if (rotate.y < -60)
        {
            rotate.y = -60;
        } else if (rotate.y > 45)
        {
            rotate.y = 45;
        }

        cam.fieldOfView = viewSize;
        Cursor.visible = visiable;
        Cursor.lockState = lockMode;
    }

    void LateUpdate ()
    {
        Transform self = this.transform;
        Vector3 targetPos = target.position;
        targetPos.y += height;

        //旋转y轴，左右滑动
        Vector2 v1 = IMath.CalcAbsolutePoint (rotate.x, radius);
        self.position = targetPos + new Vector3 (v1.x, 0, v1.y);

        //相机的观察点
        Vector2 v2 = IMath.CalcAbsolutePoint (rotate.x + defaultAngle, radius);
        Vector3 viewPoint = new Vector3 (v2.x, 0, v2.y) + targetPos;

        //计算2点之间的距离
        float dist = Vector3.Distance (self.position, viewPoint);

        //取中点作为旋转轴
        Vector3 center = Vector3.MoveTowards (self.position, viewPoint, dist / 2);

        //这里我不知道怎么计算这个任意轴，暂时先用这个方法替代
        tr.position = center;
        tr.LookAt (targetPos);
        tr.eulerAngles += new Vector3 (0, 0, rotate.y);

        //旋转x轴，上下滑动
        Vector3 temp = tr.right * dist / 2;
        self.position = tr.position - temp;
        self.LookAt (tr.position + temp);

    }

}

public static Vector2 CalcAbsolutePoint (float angle, float dist)
{
        angle += 90;
        dist = -dist;
        float x = dist * Mathf.Cos (-angle * Mathf.PI / 180);
        float y = dist * Mathf.Sin (-angle * Mathf.PI / 180);
        return new Vector2 (x, y);
}