"they have almost the same function routines and variables" (my emphasis)

Clearly the key word is "almost"
As you've given us absolutely no clue whatsoever about the nature if your projects - let alone the differences between them - it is completely impossible to help you!

You have the code - take a look at it, examine the differences, draw your conclusions!

Are you sure that all the build options are identical for both projects - particularly memory model, optimisation, and overlaying.

"i carefully declared my global variables in Xdata memory"

What about all your other variables?

"i don't know where to start"

Start at the Map file (or "Linker listing" file) - it shows you exactly what is going into XDATA, and where.

Have you read the section in the Keil C51 Manual about writing optimum code?