The optical parametric amplification of femtosecond pulse at high pumping intensity is numerically simulated by computer.The effect of cubic nonlinearity on conversion efficient of the high intensity pump pulse and the envelopes of signal pulse are studied in detail. A suitable phase mismatch is introduced to compensate the nonlinear phase mismatch induced by cubic nonlinearity. The compensation can counteract the modulation on the signal pulse on count of cubic nonlinearity and prevent the energy converting from the signal back to pump in three wave interaction. So, higher optical conversion efficiency and better quality of amplificated signal can be gotten. An explicit explanation with cascaded nonlinear phase shift is presented.