Abstract: The pressure vessel design scheme was completed based on the structure features and function requirements for the step-type control rod position sensor of Tianwan Nuclear Power Plant. The pressure vessel was mainly formed by the upper head, the cylinder body and the lower head. According to standards and working conditions, the strength calculation and the opening reinforcement of the vessel body device for cylinder body, the upper and lower head was completed. At the same time, the strength calculation of the non-standard flange supports was completed, and then the verification of the vessel was completed based on the results of strength calculation, to ensure that the vessel can form a pressure boundary and seal the working medium to achieve the purpose of maintaining the working pressure and temperature.