On 12 May 2008, the Mw 7.9 Wenchuan earthquake occurred at the eastern margin of the Tibetan plateau along the Longmen Shan fault. Based on previous geological and geophysical studies and offset maps from Advanced Land Observing Satellite/Phased Array type L-band Synthetic Aperture Radar ALOS/PALSAR data, a layered crustal structure and a five-segment rupture model are established to study the coseismic deformation of this event. Using the constrained least-squares method, a preferred coseismic slip model is derived from Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data. In particular, the Helmert variance component estimation (HVCE) method is used to determine the relative weight ratio between the GPS and InSAR data. In contrast to the initial weight ratio (0.752/0.246/0.002) among three-type observations (GPS horizontal, GPS vertical, and InSARLOS displacement), the final optimal weight ratio is found to be 0.473/0.279/0.248, and the overall root mean square (rms) misfit for all three datasets decreases from 9.15 cm without HVCE to 5.46 cm with HVCE. As revealed by the preferred slip, the slip changes obviously along the strike direction from southwest to northeast and exhibits four asperities close to Hongkou town, Yuejiashan town, Beichuan County, and Nanba town, respectively. Two major asperities are located on the Hongkou and Beichuan segments with the maximum slip close to 10 m. The slips of Hongkou, Yuejiashan, and Beichuan segments are dominated by the thrust movement with a significant right-lateral strike-slip component. The Qingchuan segment experiences a dominant right-lateral strike slip with an average magnitude of 1–2 m. Most of the slip asperities are shallower than 10-km depth, except for the southwest part near the hypocenter where the slip may exceed 20 km. The seismic moment of the Wenchuan earthquake is 8.19×1020 N m (Mw 7.91) based on the layered crust model, which is slightly larger than that using a corresponding homogeneous crust model (Mw 7.87).