Compatibility is a great matter of concern in field of Global Navigation Satellite Systems. This paper analyzes the traditional methodology of compatibility assessment based on effective carrier-power-to-noise density ratio. Meanwhile, a more accurate compatibility assessment model that includes two new factors, Pseudo-Random-Noise codes and Doppler offset, is suggested. Then, the simplified calculation for this improved model with good balance of computational efficiency and accuracy is presented. Finally, using this accuracry model with simplified calculation, the simulation results of compaitibility between Galileo and Compass on E1/B1 band are given.