The containment of nuclear power station is an important boundary of safety defense. It is very important to ensure the structural integrity and safety of the sampling pipeline when penetrating through the containment. In order to ensure the airtightness and integrity of the containment,expansion joints will be arranged in the mechanical penetration of the containment. However,the expansion joints arranged on the mechanical penetration of the containment will have an effect on the stress calculation of the sampling pipeline;And for different types of mechanical penetration,the type of connection of the expansion joint arranged on it is different,therefore,the influence on the stress calculation of the pipeline is also differ-ent. The methods of stress analysis and assessment by RCC-M code were introduced when welding the expansion joint of the sampling pipeline and mechanical penetration of the containment with focus on the research of the influence of expansion joint arrangement on the A-type mechanical penetration on the stress analysis of the sampling pipeline,and the optimization scheme was given when the pipeline fails to meet RCC-M after taking into account the influence of the expansion joint on pipe stress calculation. The present research provides certain reference for the designers of nuclear power stations.