Abstract: Control rod drive mechanism (CRDM) lower Canopy welding zone could leak under the influence of certain factors in some pressurized water reactors. The plugging assembly mounted on the welding externally can prevent and seal the leakage. According to the structure characteristics of the components, the stress conditions of key components were analyzed and the plugging effect of the assembly was verified by using the finite element software. The results show that the component can effectively reduce the stress of welding zone. At the same time, the stress level trends due to different pre-tightening forces, contact surface angle and friction were analyzed, so that the relationship curves and the optimum parameters were educed. The analysis data can be applied to the structure design, manufacture and assembly process, for making the component to achieve the best effect of plugging.