Abstract: A mechanism model was proposed to explain the infrasound abnormality observed just several days before a strong earthquake. The model includes a layered rock geo-structure, air media and their interaction. A transient dynamic finite element analysis was carried out to simulate the formation of the infrasound. The waveform and frequency of the simulated infrasound were analyzed. The results show that the proposed model is able to produce infrasound and its frequency which agree well with that from the observed. The frequency is inversely proportional to the expansion-contracting period of the earthquake source body. The fluctuating range of the air pressure during the contracting is greater than that during the expansion.