Abstract It is not rare to observe subduction zones transferring into collision zones. However, it is poorly understood the lateral variation in flexural deformation. Constrained by newly published seismic reflection data, we investigate the mechanical behavior of lithosphere from oceanic subduction to collision at the northern Manila trench using the 3‐D flexural modeling. Results show that both plate rigidity and the loadings increase from the Manila subduction zone to the Taiwan orogenic belt. In order to fit all observations, an upward loading was required at the transition zone from subduction to collision, which may be related to the variation of buoyancy due to density contrast of plate from south to north. The resultant maximum bending stress is consistent with the concentrated seismic activity at the southwest of Taiwan, suggesting that huge loading from the orogeny and lateral density change of subducting plate play important roles in bending and seismicity.