Abstract: The water-ingress accident resulted from a double-ended guillotine break of two steam generator heating tubes is a typical beyond design basis accident of HTR-PM which may cause the aggregation of hydrogen in the cavity, and consequently result in the threat of deflagration or even detonation. The paper simulated the behavior of gas entering the cavity after the double-ended guillotine break of two heating tubes, using the GASFLOW program. Besides, COM3D program was used to analyze the combustion of hydrogen in the cavity. According to the simulation results, the concentration of hydrogen in the containment is quite small, and the maximum value is below 0.1%, which means that there will exist no risk of hydrogen explosion in the HTR-PM when the water-ingress accident happens.