Whether external or internal forces of the Earth control the behaviors of upper-crustal faults in a fold-and-thrust belt has been debated for decades. The Longmenshan thrust belt (LTB) along the eastern margin of the Tibetan Plateau may provide insights into such a debate. In this study, we focus on the central segment of the LTB which has relatively uniform shortening strains yet various fluvial incision capability along the strike. This tectonic setting enables a better assessment of the effects of external forces on fault activities. We analyzed the variations of the topography, fluvial incision intensity, co-seismic slips, and co-seismic landslides along the central LTB. The Longmen sub-segment in the northern half has higher elevation and three times lower fluvial incision intensity than the Hongkou sub-segment in the south. We calculated the topographic stresses on the faults ruptured during the 2008 Wenchuan earthquake and found topographic introduced normal stress increase may explain the co-seismic slip partitioning onto two sub-faults along the Longmen sub-segment. Our results indicate that fluvial incision may have produced the along-strike variations of the topography, which may further produce the different rupture behavior. In addition, the mean hillslope angle along the central LTB prior to the 2008 Wenchuan earthquake appeared to be at the critical condition of this region. Co-seismic deformation reduced the mean hillslope angle significantly, indicating that geomorphic indices may vary with different stages in an earthquake cycle. Therefore, scrutinizing the mean hillslope angle and other geomorphic indices may help identify potential seismic hazards in an active fault system.