using UnityEngine;
using System.Collections;

public class FollowWorldObj : MonoBehaviour {
 [SerializeField]
 GameObject worldPos;//3D物體（人物）
 [SerializeField]
 RectTransform rectTrans;//UI元素（如：血條等）
 public Vector2 offset;//偏移量

 // Update is called once per frame
 void Update () {
  Vector2 screenPos=Camera.main.WorldToScreenPoint(worldPos.transform.position);
  rectTrans.position = screenPos + offset;
 }
}

using UnityEngine;
using System.Collections;
using UnityEngine.EventSystems;

public class UI_FollowObj : MonoBehaviour {
 [SerializeField]
 Camera UI_Camera;//UI相機
 [SerializeField]
 RectTransform image;//UI元素
 [SerializeField]
 GameObject obj;//3D物體
 [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;
  }
 }
}