This paper proposes an evaluation approach for an earth gravitational model(EGM), which is based on the equigeopotential surface character of the geoid. We select a gravity geoid as reference surface, calculated the gravity potentials of different EGMs on the reference surface, to build an index of gravity potential standard deviation to evaluate EGMs. We evaluated the theoretical ability for compliance of different EGMs and the same EGM applied in different areas of the earth using the approach. The results show that the geoid precisions of EGM96 and OSU91A are respectively ±11.1 cm and ±14.3 cm, meaning that the precision of EGM96 is better than that of OSU91A, while the EGM2008, EIGEN-6C4 are similar in precision. These are consistent with the existing research conclusions. But, the result further suggests that the precision of EGM2008 is slightly better than that of EIGEN-6C4 at the cm level.