Contrast to the traditional temporal phase shifting interferometry (TPSI), the simultaneous phase shifting interferometry (SPSI) immunes to the time variant error sources such as the vibration but suffers from some new systematical errors caused by the light splitter and polarization phase shifting structures adopted in most SPSI systems. A method is proposed to correct the systematical error in the SPSI, which tests a spherical surface in the SPSI system and obtains the circular carrier interferograms by defocusing, then the power and phase information of each phase shifting interferogram can be calculated using the demodulation method of circular carrier. Guided by these information the systematical errors containing the light splitter, the inaccurate phase shift and the mismatch of interferograms can be corrected respectively. This method is validated by the numerical simulations and experiments, which is suitable for all the SPSI system with light splitter structure.