Abstract: NHR-Ⅱ (Nuclear Heating Reactor-Ⅱ) developed by Tsinghua University uses the integrated gas-steam pressurizer. A calculation model of the gas-steam pressurizer was developed and validated by comparing with experiment data about the pressure behavior of gas-steam pressurizer during the in surge period. The results indicate that this calculation model can predict the transient behavior of the gas-steam pressurizer well. Then the transient characteristics of the integrated gas-steam pressurizer in NHR-Ⅱ were studied by this model. It is founded that in the power regulation of NHR-Ⅱ, the change of pressure and temperature in the system lags behind the power change in natural circulation loop with the integrated gas-steam pressurizer, and there is a sharp decline in the subcooled temperature of the core outlet. The core outlet subcooled temperature of NHR-Ⅱ is within the safety range and the system is safe in the power step regulation.