Abstract: By a least-squares fitting method we calculated the coefficients of spherical harmonic functions， and examined electron density in and around epicentral region of the Wenchuan M_S8.0 earthquake based on COSMIC occultation data. At the same time we evaluated the maximum electron density using the principal component analysis, and deduced the temporal variation of the contribution of each principal component. It was found that the enhanced anomalous TEC disturbances over seismogenic zone took place before the Wenchuan earthquake in the layer from altitude of 300 km to 450 km, further more the disturbance changed with height. The third principal component increased significantly during 19:00—24:00 (local time) within 13—27 days (from April 15 to 29) prior to the Wenchuan earthquake. The results showed that there was a clear ionospheric anomaly before the Wenchuan earthquake. Result of this study helps us in understanding the seismic-ionospheric coupling model.