The moment magnitude (MW) 7.8 earthquake occurred along the Mentawai seismic region of Sunda Trench on 25 October 2010, which is classified as the tsunami earthquake. The GPS inversion results under the assumption of simple horizontal layered medium show that the slip near the trench is not apparent, which is inconsistent with our understanding of tsunami earthquake. Here, we construct a spherical-earth finite element model (FEM) to investigate the coseismic slip distribution of the 2010 Mentawai earthquake by geometrically combining the subducting plate with a precise subduction interface. The FEM-based coseismic slip distribution shows that there are three major slip patches on the fault interface, one is located on the shallow region and the other two are located on the deeper part of the fault plane. The largest slip patch is located near the trench with the depth less than 10 km, and the maximum amplitude is about 12 m. This significant near-trench slip is consistent with the tsunami earthquake studies. A more realistic domain of FEM is used to change the coseismic slip distribution, thus significantly improving the fitting degree of the data model. These findings suggest that the FEM-derived Green's function is essential to image a more robust and realistic coseismic slip distribution of the large earthquake in the subduction zone.