Through the studing on the broadband digital seismic data from Gaotai,Lanzhou,Chengdu and Anxi seismic stations,the spectral variation of low-frequency wave before the west to Kunlun Mountain Pass earthquake with MS=8.1 in 2001,Wenchuan MS8.0 earthquake in 2008 and Minle-Shandan MS6.1 earthquake in 2003,is analyzed,and the characteristics of short-impending term precursory anomaly in the data is summarized in frequency domain.I t is found that the amplitude of frequency in 0.1~0.14 Hz increases gradually in a few days before strong earthquake.The mechanism of the anomaly is discussed also combined with tectonic features of the earthquake source area and the low-frequency wave is considered has relation with quiet-earthquake and slow-earthquake.