To study the phase- shifting characteristic of retarder array in simultaneous phase- shifting interferometry, the relationship between phase shift and fast-axis azimuth error is deduced through Jones vector and Jones matrix, and the error propagation formula is given. A spherical mirror is measured by simultaneous phaseshifting interferometer based on retarder array without calibrating fast-axis azimuth accurately and Zygo GPI XP interferometer, the difference of root-mean-square and peak-to-valley are 0.003l and 0.033l respectively. The influence of azimuth error tolerance and phase shift error of retarder array and optical axis azimuth error of polarizer is discussed.