After googled, I still can't found a way to scroll up the output of grub2 shell. However, there's a way to pause output (like the behaviour of
more command) - the
pager environment variable.
If set to ‘1’, pause output after each screenful and wait for keyboard input. The default is not to pause output.
set pager=1 command first in grub2 shell, then the output will behave like