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, as the central segment of the LTB 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 activity. We analyzed the variations of topography, fluvial incision intensity, coseismic slips, and coseismic landslides along the central LTB. The Longmen subsegment in the northern half has higher elevation and lower fluvial incision intensity than the Hongkou subsegment 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 coseismic slip partitioning onto two subfaults along the Longmen subsegment. 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 appeared to be at the critical condition prior to the 2008 Wenchuan earthquake and reduced significantly by the earthquake, 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.