Abstract: Taking the Moroccan desalination projects (NHR-10) developed by Tsinghua University as the prototype, the structure of the upper gas space was analyzed and optimized while the nuclear power maintained and the design pressure increased. The integrated natural circulation nuclear heating reactor was modeled by the RETRAN-02 code. By simulating the transients, including the inadvertent opening of the safety valve, the broken pipe outside pressure vessel accident and the load tracking, the effect of the upper gas space volume on the performance of the nuclear heating reactor was discussed. The results indicate that the total quantities of the water loss increase with the upper gas space volume in the inadvertent opening of the safety valve and the broken pipe outside the pressure vessel accident. While in the load tracking transient, much smaller volume of the upper gas space may cause higher pressure in the reactor and affect the reactor safety. Considering the operating and safety requirements of the nuclear heating reactor, the reasonable volume of the upper gas space is 1.2 m³.