I unpacked a upx file manually.
Then I want to repack it with petite,
but it does't work,why?

Did the unpacked file work in the first place?

One good guess would be that the imports of your unpacked exe have hardcoded adresses (i.e. not fully rebuilt IAT), which could cause this kind of behavior.

The unpacked file can work smoothly,but it can't be
repacked by petite2.2.When it be repacked,then it
can't work.

Can you explain the meaning?
thanks.

When you dump an app from memory, the import data might be in a state that makes it not resolve the imports again when the dumped exe is being loaded from disk, but it rather uses the exact same addresses that were resolved for the imports in the dumped app instead. This might confuse a packer in the way you describe.

Anyway, read some info about dumping and import rebuilding, and it will all become much more clear.