In this paper, coseismic GPS data and the multisurface function are first used to calculate the coseismic surface strain caused by the M_S8.0 Wenchuan earthquake that occurred on May 12, 2008 based on a piecewise data fitting method. The accuracy of the calculations is assessed, and the variation characteristics of surface strain related to the earthquake are analyzed. Calculation of coseismic strain changes revealed that the strain results obtained by piecewise fitting way are more precise than those determined by overall fitting. The calculation results of the two fitting methods feature the same order of magnitude, and the spatial distributions of the two images show some similarity. Compared with the results of the overall fitting method of multidimensional functions, the coseismic surface strain expansion and compression zones determined by the piecewise fitting method are more concentrated in the spatial distribution image. The results of this paper were compared with those of numerical simulation, and findings indicated that the calculation results of piecewise fitting are reliable. Coseismic strain appears to reflect the energy release characteristics and ground destruction distribution of the Longmenshan fault. The Wenchuan earthquake caused intense crustal shortening in the northern region of the Longmenshan fault zone. After the earthquake, the Longmenshan sub-block and the Sichuan basin sub-block moved in opposite directions with high speed over a short period of time. In the near-field region, several major aftershocks occurred in some areas where the coseismic horizontal displacement is amplitude. The coseismic surface strain highlights the right lateral slip characteristic of Wenchuan earthquake and the characteristics of uneven compression at the northwestern side of the Longmenshan fault zone are demonstrated. The results show different strain attenuation characteristics at both sides of the Longmenshan fault. Taken together, findings in this study may be used to determine the characteristics of horizontal crustal movement caused by the M_S8.0 Wenchuan earthquake in a more intuitive manner.