Abstract: The coning error owns new existing form and characteristics in gyroscope-free strap-down inertial navigation system (GFSINS).For GFSINS cannot directly give out angle increment when adopting four sub samples equivalent rotation vector algorithm,the getting method was designed and optimized to improve the efficiency of gesture updating.Finally,the coefficients in the calculating formula of equivalent rotation vector was optimized by chaos particle swarm optimization algorithm.Simulation result shows that it can effectively eliminate the influence of coning error and improve the navigation precision.