［Purposes］ The paper aims to explore the problem that the traditional reliability calculation method is not applicable to the reliability analysis of service structures.［Methods］ The improved conditional probability method was proposed considering the truncation of correlation variables and the effect of proof-load. The proposed method was applied to analyze the structure reliability of a typical non-overflow section of a concrete gravity dam in service. The dynamic reliability of anti-sliding stability of the dam foundation surface was calculated considering the resistance of structure and the load time-varying model.［Findings］ The anti-sliding stability reliability of the dam foundation surface decreases with the increase of service time, and the reliability index of structure can be improved by considering the influence of proof-load.［Conclusions］ Due to considering the effect of the proof-load and the truncation of correlation variables, the improved conditional probability method is more in line with the actual situation of the project than the classical reliability calculation method, which can provide some reference for predicting the remaining life of the service project.