Abstract: The spectrum of the signal pulses with big chirped coefficient distrusts linearly around the central wavelength in the temporal domain, which characters has been used in the numerical simulation methods, namely the fourth order Runge-algorithm, to study the OPCPA system operating in saturated amplification case. We take the LBO crystal as the nonlinear gain medium in the numerical simulation. The results showed as follows. The nonlinear phase shifting of the signal pulses occurred during the optical parameters process distrusted linearly around the central wavelength fraction. There exists a quasi-periodic structure in the power conversion efficiency, which originated from the phase mismatch among the pump pulse, signal and ideal pulse. And the maximal conversion efficiency occurs at the f