Abstract: Compatibility becomes a matter of concern in GNSS field for many countries as multiple systems like GPS, GLONASS, Galileo and Compass are all in service. In December 2010, China launched the seventh Compass Phase II satellite, and the nation plans to make it provide global service before 2020. Since Compass's signal structures are similar to and its frequencies are close to GPS, it certainly causes the interference problem. This paper analyzes the important parameters for compatibility which are spectral separation coefficient, code tracking spectral sensitivity coefficient, and effective carrier-power-to-noise density ratio. Meanwhile, the effect of code length, code-chip rate and navigation data rate onto compatibility computation is assessed. Then, a modified compatibility assessment model with good balance of computation efficiency and accuracy is suggested. The simulations of Compass and GPS signals on all navigation bands are accomplished. In these simulations, frequency distribution, signal structure definition, transmission power and bandwidth, antenna gain, elevation angle, Doppler offset, atmospheric loss and receiver’s parameters are considered. The result of simulation shows that GPS induces more interference onto Compass than that Compass induces onto GPS when full Compass constellations are deployed in 2020.