In this paper,the scale parameter S and magnitude parameter G are defined to describe gravity changes before earthquakes. 89 earthquakes with M_S ≥ 4.0 are collected to analysis the relationship between earthquake magnitude and S,G. The reference quantitative indexes for earthquake prediction are constructed. The results show that in the work of earthquake prediction,for an earthquake with potential magnitude of M_S5.0,M_S6.0,M_S7.0, or M_S8.0,the index G will be 50μGal,70μGal,90μGal,or 120μGal respectively,and the region of gravity change should be 140km,220km,350km,or 660km respectively. In addition,due to the limits on measurement interval,it is difficult to analyze the relationship between earthquake magnitude and time process of gravity changes.