Hello @maoxin
I believe what you are looking for is the functionality of the source_newgen.f routine. You will need to compile an executable and properly use it in Flair, along with a file that lists the phase space variables you want to use.
I suggest to start with the source routine lecture from the last FLUKA beginners course. Please then have a look at this post on the use of the source_newgen.f routine to read the phase space file, or this post on how to verify the phase space file is read correctly.
Hope this helps,
Andreas