For the purpose of anti-vibration for interference measurement of flat surface, a method of simultaneous phase shifting interferometry is proposed in this paper, and the experimental device is set up. Based on Fizeau phase-shifting interferometer and using quarter-wave plate as reference mirror, the whole measurement system acquires a pair of orthogonal polarized light. By using beam splitting of the chessboard phase grating, diffracted beams of (±1，±1) orders are selected by the aperture for the same ideal grating diffracting efficiency as measured beams. The four diffracted beams pass through a phase delay array and a polarizing plate to obtain four interference patterns with phase-shifting of π/2 respectively. According to the traditional four step phase shifting algorithm, the measured wave front is restored. The influence of light intensity distortion and phase shift error on the system measuring error is analyzed. It can simplify the structure of the system, reduce the system error, make installation the easier and the measurement more suitable for the plane, by comparing to the Twyman simultaneous phase shifting interferometry.