(2D intersection+picture)How to show up the invisible object when intersect with another object.

edited the picture description. Change "A is behind B " to “A is in front of B”

I have already known how to use basic overlap function such as ground check.

What you’re looking for is a Stencil mask. I wrote an example here: Using the Stencil Buffer in Unity Free | Alastair Aitchison