In this paper, we investigated the variations of the ionospheric Vertical Total Electron Content (VTEC) derived from the ground-based GPS observations during the 12 May 2008 M_s8.0 Wenchuan earthquake. A mean of the previous 10-day VTECs and the associated two standard deviations are utilized as a reference for identifying the ionospheric anomalies. The analysis showed that (1) the anomalous enhancements of VTEC appeared on May 3 and 9, while the anomalous decreases appeared on April 29 and May 6; (2)all of the ionospheric anomalies lasted beyond 4 hours and drifted from east to west; (3)relative to a non-perturbed state of the ionosphere, the zones of the VTEC decrease or enhancement by more than 30% around the epicenter had spatial scale of more than 4000 km in longitude and 1500 km in latitude at five peak times of the ionospheric anomalies. In view of the solar-terrestrial environment and the characters of the ionospheric anomalies, we think that the positive anomalies of VTEC in the afternoon of May 3 and on May 9 are probably the ionospheric precursors of Wenchuan earthquake.