Fine, the problem is my mount point parameter is error.
My pack pak file has same folder structure of main pak.
And main pak is mounted when game initialize.
I add break point when game initialize and then see FPakPlatformFile::MountAllPakFiles is loading the initial pak.
And FPakPlatformFile::MountAllPakFiles use empty mount point.
So I follow the same empty mount point to mount my pack pak, then I can load blueprint class from pack pak successfully.
But the question of replacing loaded asset is still here.