A large-scale monolithic model is established considering mainly the topographical influence nearby the project, and with stress measured data in Pubugou Hydropower Project. So the initial geostress field is back analyzed firstly by genetic algorithm and finite element method. On the basis of considering influences of some main geological structures, the refined sub-model used in the stability analysis of the caverns is accomplished, and the more precise initial geostress field of the sub-model is obtained by the second finite element analysis iteratively using displacement boundary conditions. The results show that: the first inversion is simple, with considering mainly the topographical influence and eliminating the stress distribution of the borders, its results reflect the distribution law of the large-scale initial geostress; by the second calculation considering some main geological structures, the initial geostress of the detailed structure can be obtained, and the error range is reduced bythe second calculation. The more accurate initial geostress field can be obtained efficiently by this approach as a foundation for the engineering design and construction.