Abstract: Aiming at the weld overlay repair (WOR) for the upper Ω seal of the control rod drive mechanism (CRDM), numerical simulation is applied in the integrity analysis of the overlay repaired structure. 2-D axisymmetric Gauss heat source equivalent input was built according to the welding parameter, and the weld overlay process was simulated by using 'kill elements' technology in ANSYS program, obtaining the welding residual stress of the structure. Transient stresses ware computed considering all transients in the plant, and then the fatigue results ware analyzed. Fracture mechanics analysis was also accomplished using the welding residual stress results and transient stresses results. The analysis results show that the fatigue result, the stress intensity factor and the fracture mechanics results meet the requirements of the standard.