So I found this link accidentally
https://unity3d.com/cn/learn/tutorials/topics/best-practices/fill-rate-canvases-and-input
with this "cool" part:
‘Raycast Target’ enabled by default on Image/Text
https://unity3d.com/cn/learn/tutorials/topics/best-practices/fill-rate-canvases-and-input
with this "cool" part: