In this paper, we adopt two sets of 10 Hz YRY 4-component borehole strainmeters and one set of 100 Hz FBS-3B broadband seismometer to study the 11 April 2012 Mw8.6 earthquake off the west coast of northern Sumatra, and compare the response characteristics during the coseismie stage after preproeessing the data. We study the spectrum＇s dynamic process during the coseismic stage with the S transformation method, and analyze on the spectrum＇s details of different seismic phases. The S transformation results of collocated seismometer and borehole strainmeter show that the two eoseismic spectrum evolutions have much in common and can help each other to identify the wave signals, so it＇s reliable to observe the data at high frequency using the borehole strainmeters.