For optimal SINS that both gyroscope and accelerometer were skewed installation, based on the system calibration theory of error separation, building virtual platform coordinate frame, through cosine matrix (DCM) transformed from SINS frame to platform frame and error parameter transformed from Carrier frame to platform frame, the system calibration of skewed installation optimal SINS was realized. The simulation results show that the algorithm can effectively established the transformation from SINS frame to platform frame, achieved high identification accuracy of SINS error parameter and reduced the requirement for platform positioning accuracy. So this algorithm has important reference significance for actual engineering application.