The main errors considered during online calibration of strapdown inertial navigation system (SINS) include bias error, scale factor error, installation error. The more the error parameters, the difficult calibration process, and the rocket can not do line movement during calibration stage before launch, which let the most of the error parameters unobservable. If the main parameters affecting navigation accuracy can be determined, the calibration algorithm and the calibration process can be simplified, the efficiency of calibration is improved. Based on this idea, the rotation projectile was compared to the rotation INS, starting from the error equation, the errors on the flight onboard INS was analyzed by extracting the DC component, the main parameters affecting navigation accuracy were drawn. Then a way was designed by using the speed error change around motorization maneuver. The differential equations needn′t be solved, the amount of computation was reduced greatly. Simulation results show that simple calibration scheme solves the problem of lack of incentives during launch preparation stage, and the problem can not be effectively calibrated.