Short period digital seismic data of Lanzhou seismic station in Chinese digital seismic network are used to study variation of wave spectrum parameters of small earthquakes in Yongdeng area before and after the Yongdeng M_S5.8 earthquake in 1995.The results are the followings:
(1) About two years before the Yongdeng earthquake,corner frequence f_C of S_C wave spectrum of small earthquakes in Yongdeng area fall from 2.4 Hz to 1.8 Hz.
(2) Ratio of spectrum attenuation slope in high frequency between S_g wave and S_C wave increased since 1990, then decreased about 16 months before the Yongdeng earthquake. During the ratio decreasing,the earthquake occurred.After the shock,the ratio value recovered gradually. (3) Anomalies of Q value from spectrum parameters of S_g wave appeared before and after the Yongdeng earthquake.