I install cyberpunk 2077 on my portable ssd,and i want to let the save file in the ssd,too.

From an elevated command prompt (Windows + X then A is one way) type this

mklink /D G:\SSDSaveFolder "%USERPROFILE%\Saved Games\CD Projekt Red\Cyberpunk 2077"

If you don't run under admin then you'll need to replace "%USERPROFILE%" with C:\Users\{Your Username Here}

Effectively, you're making a symbolic link to the directory in "G:" where the save files are. You'll need to rename Cyberpunk 2077 before doing this (cannot create a new link over an existing directory)

I just did that, and it's the other way round...

mklink /D "%USERPROFILE%\Saved Games\CD Projekt Red\Cyberpunk 2077" G:\SSDSaveFolder
(put G:\SSDSaveFolder in quotation marks as well if the path contains spaces)

Using symbolic links will definitely work (it's a very useful trick, solves many issues and annoyances especially with save file locations in games) but I think you may need to create that symbolic link on every system/account that you will be playing on. You will also need to correct the path to your portable drive each time it changes.