I finally figured it out. it’s just a careless mistake. My code is:
FollowCamera->SetupAttachment(RootComponent,USpringArmComponent::SocketName);
which is wrong because the first parameter shouldn’t be RootComponent, it should be the SpringArmComponent, as we want to make the camera be SpringArmComponent’s child so that the camera can move along with the spring arm.
But some wierd things happened, after I corrected my code, the chararcter blueprint class, which was created based on my C++ class, didn’t change. the camera was still not the child of SpringArmComponent. So I deleted bluepring class and recreated it. Finally, it changed. Now I know that the blueprint class will not totally update with its C++ code…