In　2019,　China　revised　and　issued　the　national　standard　＇standard　for　seismic　design　of　nuclear　power　plants＇　(GB　50267-2019),　with　the　new　clause　that　the　earthquake　monitoring　and　alarm　system　should　operate　reliably　in　peacetime　and　in　case　of　design　basis　ground　motion,　and　the　earthquake　alarm　index　should　be　fully　demonstrated.　The　peak　ground　acceleration(PGA)　is　widely　used　as　an　earthquake　warning　index　in　China,　However,　from　the　three　elements　of　ground　motion,　it　only　considers　the　amplitude　characteristics,　and　not　consider　the　influence　of　frequency　and　duration.　It　is　likely　to　cause　false　alarm　and　affect　the　reliable　operation　of　NPPs.　According　to　the　principle　of　three　elements　of　ground　motion,　this　paper　proposes　a　method　to　select　the　filtering　frequency　band　which　is　in　line　with　the　structural　characteristics　of　NPPs　for　filtering,　then　synthesize　the　triaxial　acceleration　time　history,　and　finally　select　the　peak　acceleration　with　appropriate　duration　for　threshold　alarm.　Relevant　improvement　suggestions　are　also　put　forward　for　automatic　seismic　trip　system(ASTS).　©　2022,　The　Author(s),　under　exclusive　license　to　Springer　Nature　Singapore　Pte　Ltd.