The 1500-km-long Jiangnan tectonic belt in China is well known for large outcrops of Proterozoic basement rocks, although the exhumation history of the exposed basement is poorly constrained. This study performed thermochronology of the Sanfang batholith in the southernmost section of this belt to constrain the timing and mechanism of exhumation. Granite samples collected along three transects were subjected to apatite fission-track analysis. The results show that the ages of single apatite grains are much younger than the Neoproterozoic crystallization age of the Sanfang batholith. The apparent age versus elevation diagram reveals two-stage cooling, during the Paleogene and Miocene. The cooling stages are coincident with unconformities between the Paleogene and underlying strata, and between the Neogene and the Paleogene, with movement on high-angle normal faults. A compilation of apatite fission-track data from this and previous studies suggest that the batholith was finally exhumed in Cenozoic and timing of rapid cooling are progressive younger to the east resulted from a slab rollback causing the Cenozoic back-arc extension. Thus, exhumation of the Sanfang batholith indicates that Cenozoic erosion and extensional tectonics played a key role in exhuming the basement rocks and forming the present architecture of the Jiangnan tectonic belt.