1.当Canvas.RenderMode为Screen Space-Overlay时

利用WorldToScreenPoint(worldPos)将物体的世界坐标转换成屏幕坐标,实时更新UI的坐标：

using UnityEngine;

using System.Collections;

public class FollowWorldObj : MonoBehaviour {

        [SerializeField]

        GameObject worldPos;//3D物体（人物）

        [SerializeField]

        RectTransform rectTrans;//UI元素（如：血条等）

        public Vector2 offset;//偏移量

        void Update () {

                Vector2 screenPos=Camera.main.WorldToScreenPoint(worldPos.transform.position);

                rectTrans.position = screenPos + offset;

        }

}

2.当Canvas.RenderMode为Screen Space-Camera时

利用RectTransformUtility.ScreenPointToLocalPointInRectangle换算出UI元素在Canvas的2D坐标:

using UnityEngine;

using System.Collections;

using UnityEngine.EventSystems;

public class UI_FollowObj : MonoBehaviour {

        [SerializeField]

        Camera UI_Camera;//UI相机

        [SerializeField]

        RectTransform image;//UI元素
        [SerializeField]

        GameObject obj;//3D物体

        [SerializeField]

        Canvas ui_Canvas;

        // Update is called once per frame

        void Update () {

                UpdateNamePosition();

        }

        /// <summary>

        /// 更新image位置

        /// </summary>

        void UpdateNamePosition()

        {

                Vector2 mouseDown = Camera.main.WorldToScreenPoint(obj.transform.position);

                Vector2 mouseUGUIPos = new Vector2();

                bool isRect = RectTransformUtility.ScreenPointToLocalPointInRectangle(ui_Canvas.transform as RectTransform, mouseDown,

                UI_Camera, out mouseUGUIPos);

                if (isRect)

                {

                        image.anchoredPosition = mouseUGUIPos;

                }

        }

}

参考：https://blog.csdn.net/nnew_hande/article/details/78501309