Based on transfer matrix method, the output characteristics of Mach-Zehnder interferometer are analyzed. Results show that Mach-Zehnder interferometer has differential characteristics when difference of two interference arms′ length is an integer multiple of the ratio of the center wavelength to the effective refractive index, and the differential order is proportional to the number of cascaded interferometers. Based on signal system theory and the differential characteristic of Mach-Zehnder interferometer, the ultrashort optical pulse synthesis system based on Mach-Zehnder interferometer is designed. By adjustment of the weighting coefficients of input Gaussian pulse and its differential order, input Gaussian pulses are synthesized into flattened pulse, triangular pulse and parabolic pulse, respectively. When the weighting coefficients and input width of Gaussian pulse are changed, the output pulse waveform is basically invariable, which proves the system has good stability.