Amplification and simultaneous phase regeneration are necessary for phase modulated optical fiber communication system. In this paper,using semiconductor optical amplifier (SOA) as nonlinear medium,a nonlinear Mach-Zehnder interferometer (MZI)as the regenerator is designed. The formula of signal after the SOA based MZI regenerator is educed. For different incident power of signal,the relations between signal-pump phase difference and signal gain are discussed. The optimum parameters of input signal and pump are obtained. The distributions of phase and intensities for both the non-regenerated and regenerated signal are simulated. The results show thatthe SOA based optical regenerator has a good regeneration.