Abstract: Air ingress accident, which will result in graphite oxidation reaction of graphite reflectors and fuel elements so as to possibly weaken the structural strength, impact the retention capacity of coated particle and produce flammable gas mixtures, is considered as one of the severe hypothetical accidents for high temperature gas-cooled reactors. The research on air ingress accident is significant for the study of the reactor accidental characteristics and the improvement of the safety design. Based on the preliminary design of the 200 MWe Pebble-bed Modular High Temperature Gas-Cooled Reactor (HTR-PM), the air ingress into the core due to the chimney effect caused by the simultaneous rupture of both upper and lower fuel discharging pipes connected to the primary loop was analyzed in detail. By the help of the HTR system software TINTE, the natural convection flow rate, core graphite corrosion rate, oxygen consumption quantity and fuel element temperatures were studied. The results indicate that the coated fuel particles will not be exposed in nearly first 60 h even considering the heterogeneous corrosion. During the accident, the maximum fuel temperature will not exceed the design limitation of 1 620 ℃, so that the integrity of the fuel particles and the ability of retaining fission product will be kept well. Besides, if the air source could be cut off to impede the continuous graphite corrosion by some measures, the reactor safety will not be endangered any more.