Abstract: The thickness of the lunar crust facing the earth is 40-60km, where most maria exist. Whereas that of the other side is up to 150km. This makes the asymmetry of the moon. The present paper put forward an explanation for the formation of this asymmetry. In the early stage, there was solid lunar core surrounded by magma “ocean”, and the lunar crust was formed by gravitational differential and cooling consolidation. Owing to the synchronous rotation with the earth, one side of the moon always faces the earth. The solid lunar core moved towards the earth by earth’s gravitational field, and as a result, the light substances on the lunar surface floated to the back side of the moon. This evolution mechanism make the lunar surface thinner in the near-earth side while thicker in the back side. Through theoretical inducement, the paper gives a formula to calculate the distance of lunar core moing toward the earth as: r=2Md×R3/My(D2-R2) Where Md is the mass of the earth, My is the mass of the moon, Ris the radius of the moon, and D is the distance from moon to earth. According to this theory, it is concluded that the distance between the moon and the earth was 162.700km when their synchronous rotation happened.