How can I get a smooth rounded shaped normal map? I'm new to blender 2.8, and practicing how to bake normal maps right now.

In this picture, I was trying to bake high poly cylinder to make a low poly cylinder look smoother.

I just follow this youtube video(https://youtu.be/0r-cGjVKvGw), but when I baked it, these weird kind of edge things appear in the normal map.

Changing ray distance or cylinder size doesn't work, what is wrong and what should I do?

p.s. sorry for bad English XD
Tangent space normal maps give transforms of normal values relative to the face normal, if the face normals are not smooth then it doesn't follow that the normal map will be either.

In this case it looks like your face normals are discontinuous on the cylinder's 'curved' edges, and thus so is the normal map. This will probably render fine.

If you do want a continuous normal map, you could set these faces to be smooth shaded on the lower poly.