At present, the utilization rate of underground space is relatively low.? In order to alleviate the problem of insufficient living space for people, a similar model test of excavation of underground space of existing buildings with additional layers down was carried out based on the project of building function as a hotel in Hangzhou commercial Center. The experiment simulated the process of support and excavation. In the process of underground layer addition of the existing building, the existing supporting structure is not enough to provide sufficient support function, so it is necessary to add the new supporting structure. In the new supporting structure, the supporting characteristics of the existing supporting pile will change significantly. Therefore, the change law of the horizontal displacement of the foundation pit slope and the bending moment of the pile body will be studied to analyze the stress mechanism of the new supporting pile and the existing supporting pile system. The experimental simulation accuracy is high; Uncontrollable parameters such as soil cohesion, friction Angle and controllable factors such as pile length and pile spacing have a great influence on the supporting effect. The model experimental data can provide references for the selection of design parameters for the downward layer addition of existing buildings. The experimental results show that the horizontal displacement of pile top of pile foundation increases with the increase of excavation depth of underground layer. The bending moment of pile body increases linearly with the excavation depth, and the bending moment of pile body has a bow distribution, that is, the middle is large and the two ends are small. Whether the existing supporting pile has crown beam or not has great influence on the robustness of the supporting system.