Abstract: The advanced integrated small reactor adopts steam-gas pressurizer with simple structure and pressure self-stabilizing ability. In order to study the thermo-hydraulics characteristics of steam-gas pressurizer, a steam-gas pressurizer model based on multi-zone non-equilibrium state was established. The multi-zone non-equilibrium steam-gas pressurizer model consists of three parts: liquid zone, saturated zone and gas (steam) zone, and each zone could be further divided into several control volumes. From the point of view of mass and energy conservation, the important thermodynamic and hydraulic phenomena in pressurizer, such as volume evaporation, volume condensation and wall heat transfer, were described. By assuming that the volume evaporation and condensation come from the rise of bubbles and the fall of condensate, the volume evaporation models and volume condensation models were established, and considering the influence of nitrogen on the heat exchanger process in the gas (steam) zone, the wall heat exchange model was modified by empirical relation to ensure that the heat exchange process could be accurately simulated, furthermore the discrete solution method of the steam-gas pressurizer governing equation was proposed. The calculated values of the steam-gas pressurizer model were compared with the boosting experimental results of the steam-gas pressurizer with different nitrogen fractions (3.2%, 9.7%, 20%) of Massachusetts Institute of Technology (MIT). The results show that, in the boosting stage, the multi-zone non-equilibrium steam-gas pressurizer model can accurately simulate the pressure response characteristics. The maximum relative error between the multi-zone non-equilibrium steam-gas pressurizer model calculated pressure value and the experimental value is about 1.93% under the 9.7% nitrogen fraction case, and about 1.60% under the 3.2% nitrogen fraction case, and 3.80% when the model calculates the pressure value and the experimental value under the 20% nitrogen fraction case. In the condensation and depressurization stage, the relative error between the calculated value and the experimental value increases due to the influence of nitrogen on the heat exchange condensation of the wall, but the relative error between the model calculated value and the experimental value of different nitrogen fraction cases is within 9%. On the whole the developed steam-gas pressurizer model can accurately predict the pressure response of pressurizer and can be used to analyze the pressure stabilization characteristics of steam-gas pressurizer.