The May 12th, 2008 Wenchuan earthquake (Mw 7.9) in Sichuan Province led to severe casualties and a huge economic loss. Field investigations after the earthquake have obtained a lot of co-seismic deformation data. In order to estimate the horizontal shortening, the authors suggested a new method: drawing reference geometric features in QuickBird images first; then calculating the space between adjacent features before and after earthquake, respectively; finally obtaining the co-seismic horizontal shortening based on the contrast of the spaces before and after earthquake.