We detected the variations of crustal seismic velocity around the M_s8.0 Wenchuan earthquake fault zone during March 2007-March 2009 from ambient noise cross-correlation by using continuous seismic data recorded by the broadband stations of Sichuan digital seismic network. The data processing procedure was divided into three phases: (1) single station data preparation including time domain normalization, spectral whitening, (2) cross-correlation and temporal stacking of 31-day daily cross-correlation functions (CCF), and (3) estimation of relative velocity change by measuring travel time shifts in frequency 0.1~0.5 Hz between the 31-day stacks and the reference empirical Green function, which was the stack of 14 months of CCF before the Wenchuan earthquake. The preliminary results revealed a sudden post-seismic velocity drop for the station pairs across the Longmenshan fault, the largest of which was more than 0.4%. The seismic velocity changes with time exhibited a spatial difference: in the southwest segment of the Longmenshan fault, the maximum value of velocity reduction occurred at the time immediately after the Wenchuan earthquake, whereas in the northeast segment of the fault velocity reduction appeared 1~4 months after the main earthquake. Furthermore, the spatial extent and magnitude of the post-seismic velocity change in the Sichuan Basin exceeded that in the mountainous regions west of the fault zone. The static stress change and near-surface physical damage caused by strong ground motion could not entirely explain the measurements in this study. We think the temporal changes of seismic velocity maybe related to the damages from fault ruptures and stress changes around the fault zones.