Abstract: In order to overcome the problems of time-consuming and complex operation caused by the traditional degassing method of chemical and volume control system, a design scheme of pressurizer degassing system using the pressurizer for thermal degassing is proposed. A dedicated computer program is developed based on pressurizer steady degassing model and optimization algorithm, by which the pressurizer degassing optimization calculation and analysis for the small module reactor is carried out in full range of shutdown operation conditions, and the size of the optimized flow limiting orifice and influence factors are obtained. The evaluation with the practical degassing effects shows that, the degassing system proposed in this paper can complete the hydrogen removing task in 4.13 hours theoretically, which is with obvious advantages over the traditional degassing method of the chemical and volume control system.