When　the　weave　of　earthquake　spreads　out　in　the　earth,　it　can　radiate　the　infrasound.　The　kind　of　infrasound　is　called　as　earthquake　infrasound.　The　frequency　of　infrasound　is　under　the　20　Hz,　it＇s　magnitude　is　declined　slowly,　and　at　the　same　time　it　spreads　far.　The　infrasound　can　spread　several　kilometers　long　in　the　air　(from　the　earth＇s　surface　upper　to　the　50　kilometers).　The　infrasound　can　be　detected　after　circling　the　earth　several　weeks.　Because　of　these　characters,　the　infrasound　has　the　good　opposite　connection　with　the　earthquake.　As　a　rule,　the　abnormal　infrasound　signal　can　be　detected　beyond　the　magnitude　7　of　earthquake　in　the　world.　Using　the　good　capabilities　of　computer　and　the　more　and　more　perfect　technologies　of　signal　analysis　to　find　out　the　inside　qualities　of　infrasound　abnormal　signal　before　the　earthquake,　which　will　provide　the　important　value　for　forecasting　the　earthquake　later.　Using　the　discrete　Fourier　transform　and　wavelet　theories,　the　abnormal　signal　is　found　and　it＇s　range　of　frequency　is　about　from　0　Hz　to　4　×　10-3　Hz　after　the　abnormal　signal　is　discrete,　at　the　same　time　the　abnormal　signal　is　symmetrical　and　regular　in　higher　scale　and　the　abnormal　signal　is　broken　in　lower　scale.