Abstract: To reveal the distribution of residual stress in P91 steel following multi-pass welding and heat treatment, this study focuses on P91 martensitic heat-resistant steel. The residual stress in post-treatment specimens was initially measured using the blind-hole method. Subsequently, a numerical simulation of the multi-pass welding process was conducted. Finite element models were employed to calculate the temperature field and residual stress field separately, followed by a simulation of the heat treatment process. The results indicate that the distribution trend of residual stress near the weld joint obtained from the simulation agrees well with experimental data. The error between the simulated and measured peak values after heat treatment was less than 15%, validating the reliability of the numerical method used. Based on this accurate model, the distribution and magnitude of residual stress before and after heat treatment were compared. This research can provide theoretical support for the rational control of residual stress in welded structures and for improving joint welding quality.