Abstract: Multiple sets of stress relaxation tests were conducted on ST460TS, ST12T and Stellite 6 alloys used in ultra-supercritical steam turbine valves, under different initial stresses and at the valve service temperature of 630 °C. The influence of initial stress on stress relaxation was investigated for various materials. The stress relaxation curves were fitted using a four-order exponential decay function, and a full-stress-range creep constitutive model based on stress relaxation was derived. Results show that ST460TS and ST12T exhibit similar characteristics under different stress levels. The entire stress relaxation process can be divided into three stages: rapid relaxation, transitional relaxation and progressive steady state. The four-order exponential decay function can accurately describe the stress relaxation process of ST460TS and ST12T, and predict the relaxation limit. Stellite 6 exhibits stepwise relaxation behavior under high initial stress due to its low plasticity, while it shows continuous decay relaxation under low initial stress.