We conducted two-dimensional physical and numerical modeling to simulate the synorogenic extension in the Taiwan mountain belt that is created by the collision between the Eurasian and the Philippine Sea plates at the Chinese continental margin. Our results show that crustal extension may occur around the crest of the Central Range because of gravitational spreading. Accompanying the extension, active thrust belts appear on either side of this extensional area. Overprinting of late-stage normal faults onto early-stage compressional structures on the east flank of the Central Range may result either from changes in the stress field near the mountain crest at various stages of the mountain-building event or from west to east compression at depth and tension near the surface. The concentration of normal faulting on the east flank of the Central Range suggests that the mantle suture between the Philippine Sea plate and the Eurasian plate might be farther west than the Longitudinal Valley. Unlike the hypothesis of mountain collapse, implying an overall reduction of elevation, our model shows that the synorogenic extension bounded by active thrust belts appears in the area undergoing fast surface uplift, as observed in Taiwan.