Considering the Alxa Left Banner M5.8 earthquake that occurred on April 15, 2015, the continuous digital seismic waveform data recorded by Wuhai, Dongshengmiao, and Shizuishan seismic stations from 00:00 April 1 to 23:00 April 15, 2015 were analyzed using Fourier transform, and the variation of maximum envelop amplitude within 0~0.25 Hz was tracked. The results reveal the following. (1) Before the M5.8 earthquake, the seismic waveform recorded by Wuhai, Dongshengmiao, and Shizuishan stations near the epicenter all show that the frequency spectrum shifts to low frequency. (2) The tracking forms of maximum envelop amplitude with low frequency anomaly in the three stations are different. Before the earthquake, only the tracking form in Wuhai station shows obvious continuous unsteady anomaly, which continues for about 120 hours. (3) The tracking forms of maximum envelop amplitude in the three stations are quite different with their distance from epicenter.