A　new　characteristic　parameter　S-d　tau　is　proposed　to　improve　the　performance　of　magnitude　estimation　in　earthquake　early　warning　(EEW).　S-d　tau　is　the　product　of　summation　of　absolute　displacement　multiplied　by　the　maximum　predominant　period　(tau(p)(max))　for　the　first　arriving　seconds　of　a　seismic　wave.　About　30,725　underground　records　at　borehole　stations　for　3645　earthquakes　with　magnitude　between　4.0　and　9.0　from　the　Japanese　KiK-net　were　used　to　compare　the　magnitude　proxy　performance　based　on　the　proposed　S-d　tau　with　that　based　on　either　tau(p)(max)　or　peak　displacement　P-d.　The　comparison　results　show　that　for　a　magnitude　between　4.0　and　7.3,　S-d　tau　has　a　better　correlation　with　magnitude　and　higher　estimated　accuracy　than　either　tau(P)(max)　or　P-d.　Hypocentral　distance　is　not　required　when　using　S-d　tau,　but　it　can　be　used　to　further　improve　the　accuracy　of　magnitude　estimate.　These　results　confirm　that　S-d　tau　can　significantly　improve　the　accuracy　and　timeliness　of　continuous　magnitude　estimation　in　an　EEW　system.