Abstract: Many concerns are paid to the double-ended guillotine break (DEGB) of horizontal gas duct accident because of the possible serious consequence. For the 200 MWe high temperature gas-cooled reactor pebble-bed module (HTR-PM), the DEGB of horizontal gas duct accident is very important for the safety analysis of HTR-PM. The method of injecting a certain flow of nitrogen or helium from the fuel charging tube was proposed to mitigate the consequence of the DEGB of horizontal gas duct accident. By using the system code TINTE-TIIXUW, the mitigation effect when injecting different mass flows of nitrogen or helium on the consequence of DEGB of horizontal gas duct accident was researched. According to the simulation result, the mass flow of nitrogen shall reach a certain value when the nitrogen can play a role in mitigating the consequence. However, as to the helium, both the small flow and the large flow can play a role in mitigating the consequence of air ingress accident, which will be helpful to the future engineering application as a good reference of mitigation measure.