We analyse the influence of forward Raman scattering(FRS) on the wake-field induced by ultra-intense short-pulse laser with a 25D PIC simulation model. FRS is the major unstable mechanism that draws out the length of scattering pulse, as well as steeps the tail of the laser. Therefore, the wake wave phase velocity decreases obviously and the maximum electron kinetic energy trapped is much less than the conventional theoretic value. Furthermore, the frequency spectrum profile shifts to lower freguency.