The continuous recorded broadband seismographics waveform data in 12 days before the Wenchuan M8.0 earthquake in 2008 from 3 seismic stations of Sichuan digital seismic network and one in 14 days before the Yushu M7.1 earthquake in 2010 from 3 seismic stations of Qinghai digital seismic network, which nearby the epicentres, are chosen and analyzed. The result shows that dominant frequence of background noise obvious changed before the two main rupure, and both cases have similar character. It indicated possible slight ruputures in the seismogenic zone before the great earthquake. Althougt it is a primary result, it is helpful to research induction machinism of earthquake and earquake prediction explore.