Abstract: The heat and mass transfers in the interface of steam and liquid and condensation near the wall with non-condensable gas were analyzed. A steam-gas pressurizer model was developed, the process of pressure transient in the pressurizer with the non-condensable gas was investigated and the thermal-hydraulic characteristics of a steam-gas pressurizer were described. The calculation results were verified with the results from the MIT pressurizer experiment. The results show that the calculation accuracy using pressurizer model without non-condensable gas is high, the relative deviation is 0.8% and the peak pressure is 0.647 MPa. When the mass fraction of non-condensable gas is from 0 to 20%, the calculation accuracy decreases, the maximum relative deviation is 15.4% and the peak pressure is from 0.647 MPa to 1.02 MPa. It’s found that the non-condensable gas plays an important role on the pressure response in the gas-steam pressurizer. When the mass fraction and varieties of non-condensable gas are different, the pressure response in the pressurizer is significantly different.