Abstract: The navigation system precision was influenced by the output error of fiber optic gyro(FOG) inertial measurement unit(IMU). In order to complement ground calibration, FOG strapdown inertial navigation system(SINS) online calibration was carried out in airborne guidance weapon in-flight alignment process. In-flight alignment/online calibration model of FOG-SINS was introduced, based on which the Kalman filter was designed. Simulation analysis of error excitation and influence on navigation precision were carried out, and filter optimal design was completed. At last, the simulation results with or without optimal design were obtained which indicated that: FOG-SINS free inertial navigation precision was improved after optimal design.