I have a problem of normal when rendering. I made sure all normals direct outward but when I do merger of two different kinds of geometries, one turns in black. Why does Houdini do to me?

Can you post a HIP file? That'd help with figuring out what's gone wrong. If you're using a Merge SOP and one input has N, but another doesn't have N, N will be zero for the geometry that didn't have N before. In that case, there should be a warning on the Merge SOP about mismatched attributes.

Add a normal SOP in the end.