Abstract: In order to solve the problems that The satellite-ground double differential phase observation of onboard GPS is given to determine the earth gravity model from GPS phase data onboard CHAMP and observations of 48 IGS tracking sites.The Cowell II numerical orbit integration and the partitioned Bayesian least squares estimation are used to calculate the spherical harmonic coefficients of the earth gravity field to 70 degrees.The maximum difference of geoid undulation between the model and EGM96(to 70 degrees) is 2.872 m.The difference accuracy of geoid undulation is 0.522 m and the average difference is(-0.003 m.) These indicate that there is no systematic difference between the model and EGM96.