Almost　all　nuclear　power　plants　in　the　world　are　equipped　with　seismic　instrument　system,　especially　the　third　generation　nuclear　power　plants　in　China.　When　the　ground　motion　measured　by　four　time　history　accelerometers　of　containment　foundation　exceeds　the　preset　threshold,　the　automatic　shutdown　trigger　signal　will　be　generated.　However,　from　the　seismic　acceleration　characteristics,　isolated　and　prominent　single　high　frequency　will　be　generated　the　acceleration　peak,　which　has　no　decisive　effect　on　the　seismic　response,　may　cause　false　alarm,　which　has　a　certain　impact　on　the　smooth　operation　of　nuclear　power　plant.　According　to　the　principle　of　three　elements　of　ground　motion,　this　paper　puts　forward　a　method　that　first　selects　the　filtering　frequency　band　which　accords　with　the　structural　characteristics　of　nuclear　power　plants,　then　synthesizes　the　three　axial　acceleration　time　history,　and　finally　selects　the　appropriate　acceleration　peak　value　for　threshold　alarm.　The　results　show　that　the　seismic　acceleration　results　obtained　by　this　method　can　well　represent　the　actual　magnitude　of　acceleration,　and　can　solve　the　problem　of　false　alarm　due　to　the　randomness　of　single　seismic　wave,　and　can　be　used　for　automatic　reactor　shutdown　trigger　signal　of　seismic　acceleration.　©　2021　American　Society　of　Mechanical　Engineers　(ASME).　All　rights　reserved.