Gas-Assisted Gravity Drainage (GAGD), solving the problem of low volume sweep coefficient for continuous gas injection (CGI) or water-alternating-gas injection (WAG) and increasing the ultimate recovery to a great extent, is a recent method for enhancing oil recovery. However, no effective predicting methods of oil recovery for GAGD exists. Based on the study of producing mechanism of GAGD and dimensional analysis method, this paper analyzed three dimensionless varies, Capillary Number, Bond Number and Gravity Drainage Number, and evaluated its relative influence on oil recovery. Considering the effect of wettability, viscosity ratio, and the difference between oil density and gas density, Bond Number and Gravity Drainage Number are modified and its relationship with recovery is established by experiment and field data. Finally, a new predicting method of oil recovery for GAGD is set up. Application shows that this method is far more suitable for GAGD and its predictive result is more reasonable. Therefore, it is quite significant for determining recovery of such type of development mode.