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.