Current numerical models for studying the stability of underground cavern group are simplified 3D models that are only composed of the main powerhouse, the main transformer chamber, the main electrical wire hall, part of the diversion tunnel and tailrace tunnel. The influence of the excavation of other caverns is usually ignored, leading to the inconsistency with realistic project conditions. In this study, a 3D FEM model of the Panlong Power Station, considering all the caverns as well as a 3D simplified model only including part of the cavern was built. The deformation characteristics and stress states of the cavern group during excavation were studied using 3D nonlinear FEM. The results show that the simulated distributions of deformation field, stress field, and plastic zone between the two models are similar but the magnitudes of the results from the 3D FEM model considering all caverns are generally higher than that of the simplified model. The difference between the two simulated horizonal displacements of the upstream wall can reach 19. 5%. Therefore, from the point of view of engineering safety, it is recommended to choose 3D FEM simulation models considering all caverns to perform stability analysis for underground cavern group excavation.