Abstract: In order to reveal the mechanism of stress relaxation in bolted connections during long-term service, a systematic analysis of the creep mechanism and performance retention behavior of bolted connections under service conditions was conducted. By combining simplified analytical methods and finite element analysis, the stress relaxation of bolts and flanges over different service times was thoroughly investigated. A simplified analytical method was adopted to address the creep stress relaxation problem in bolted flange connections, with particular focus on the creep relaxation of flange and bolt materials. The validity of this method was verified by comparison with three-dimensional numerical finite element methods, and the influence of different service times on the decline of bolt preload was analyzed. Simulation results showed that the overall creep relaxation of the simulation model was 45.2% at 1 000 h, 50.8% at 3000 hours, and 63.3% at 6 000 h. Analytical results indicated that the overall creep relaxation of the analytical model was 53.4% at 1 000 h, 60.5% at 3 000 h, and 73.4% at 6 000 h. The results demonstrated that the longer the service time, the faster the initial decrease in preload, followed by a gradual stabilization. In summary, through detailed theoretical analysis and numerical simulation, the stress relaxation mechanism of bolted connections in long-term service is revealed, and reliable predictive methods and design optimization schemes are provided.