The fractured-cavity carbonate reservoir spaces，including pores，caves and fractures，have varied scales，irregular shapes and discontinuous distribution.Quantitative characterization of effective reservoir space is difficult，and the accurate calculation of petroleum geological reserves of the reservoir is the key to technical difficulties.Therefore，the probability method based on 3D geological model was studied and discussed.Based on the single well fractured-cavity reservoir identification and reservoirs prediction between wells，a series of geological models of equal probability were established by methods of deterministic modeling and stochastic modeling combined.According to the 3D geological model of grid data，the effective fractured-cavity reservoir volume was calculated.On the basis of pores，caves and fractures of the 3D geological model，the petroleum geological reserves were computed separately，as well as the statistical probability distribution of petroleum geological reserves.The most likely petroleum geological reserves of Tahe-IV district is 5 665×104 t，the most pessimistic is 5 470×104 t，the most optimistic is 5 914×104 t，and the calculation reserves results and the Tahe oilfield production situation is basically the same.The results show that，by means of the probability method based on 3D geological model considering the heterogeneity of reservoir，the oil geological reserves calculation is more accurate，and it can detail the composition of petroleum geological reserves，and can point out the favorable petroleum geological reserves accumulation area.