With more demanding requirements of the large scale foundation on the construction progress and ground limitation, construction of the anti-pulling pile before the foundation pit excavation has gradually become the trend. The stress state and ultimate bearing capacity of the pile will change greatly with massive unloading at foundation pit excavation. By combining the numerical simulation of the particle flow with the theoretical analysis of the limit equilibrium, the failure mechanism and ultimate bearing capacity of the anti-pulling pile unloaded at excavation is analyzed. The analysis shows that the damage characteristics of the surrounding soil is an inverted cone shape with the linear failure surface and failure angle. Unloading at excavation has little influence on the failure angle of the surrounding soil, but the influence of the pile length on the failure angle is significant. According to the research results of the mechanism, the theoretical analysis model has been established with the theoretical formula for the bearing capacity and loss ratio of the pile derived. The theoretical formula has been verified by the existing simulation test results.