The large domain model is universally used to simulate the effects of terrain, physiognomy etc. on original geostress. The small range model, in which mesh is refined, is exactly used to simulate excavation process of the underground powerhouse. The displacement boundary values of the small model use the results of the large model, which are obtained by interpolating with numerical inverse isoparametric mapping. The example and the case verify that the method of numerical inverse isoparametric mapping has high precision, and can be used in geotechnical engineering.