This paper proposes an approach to update the attitude and velocity synchronously utilizing the fourth order Runge-Kutta in the strapdown inertial navigation system. The synchro-updating algorithm is designed for the condition, in which the outputs of gyroscope are the angular rate and the outputs of accelerometer are the specific force acceleration. The synchro-updating algorithm is based on the quaternion. Under the improved classical coning motion and the generalized vibrational motion respectively, this paper analyzes and compares the attitude error and velocity error of the above algorithm with other algorithms'. The simulation results show that the synchro-updating algorithm is more effective than the others for the angular rate outputs and the specific force outputs.