On the basis of about 300 earthquake wave forms observed in the Shidian MS=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5~5.3, the seis-mic moments are 1010~1016 N?m, the corner frequencies are 0.2~0.8 Hz, radii of the focal rupture are 200~2 500 m and the stress drops are 0.1×105~20×105Pa. Through the statistical analyses of variation of corner frequency fc and stress drop ?σ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops ?σ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of av-erage corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses. On the basis of about 300 earthquake waveforms observed in the Shidian MS = 5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear waves have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5-5.3, the seis-mic moments are 1010-1016 N? m, the corner frequencies are 0.2-0.8 Hz, the radii of the focal rupture are 200-2,500 m and the stress drops are 0.1 * 105 to 20 × 105 Pa. Through the statistical analyzes of variation of corner frequency fc and stress drop? Σ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. stress drops? σ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of av-erage corner frequency and stress drops before and after the main shock have index significance to the The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of ​​main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses.