Using the D-InSAR technology, we have analyzed the Japanese satellite ALOS/PLASAR data before the 2008 Wenchuan earthquake and obtained four zones of ground deformation which cover the Yingxiu town, Maoxian county, Beichuan county, Pingwu and Qingchuan county in Sichuan Province. The result shows that before the Wenchuan event, the deformation field was a symmetric convex-shaped pattern around the fault. Vertical uplift appeared nearby the seismogenic fault, while subsidence took place far on its both sides away from the fault which increased in amplitude with increasing distance from the fault. Around Pingwu, Beichuan, Anxian and Jiangyou, a large area of uplift was present on the both sides of the fault with amplitude 5~10 cm. Nearby the Yingxiu town and the epicenter of the main shock, small scale spot-like swelling of 10~15 cm occurred separately, which coincides with the location of the epicenter as well as the most obvious rupture section. In the areas far away from the fault on both sides, there was dominantly subsidence, which was measured 0~-10 cm around Mianyang and Chengdu City and -5~-10 cm from Songpan to Wenxian, respectively. Our study indicates that along the locked thrust fault, vertical uplift is its primary characteristic before the quake. Although the derived deformation before the Wechchuan event from this work probably contains errors of 5 cm related with DEM and the atmospheric delay in the troposphere, such an abnormal change in the vertical deformation field indeed existed which can serve as a line of evidence for earthquake prediction.