Covalent organic frameworks (COFs) has become known as a promising organic photo catalyst for hydrogen evolution reaction (HER) because of its highly designable structure at the molecular level and other advantages related to the photocatalytic reaction. Here, we report a facile approach to prepare few-layer COFs nanosheets from a typical layered COF (covalent triazine frameworks, CTFs) via intercalation and exfoliation with H2SO4 and (NH4)(2)S2O8. Both the experiments and DFT calculations revealed the exfoliated CTFs nanosheets owned tunable optical and electronic properties because of the introduced defect states. It can not only narrow the bandgap for improved visible light absorption, but also suppress the radiative electron-hole recombination. Besides, the ultrathin structure can shorten the diffusion length of the photoexcited electrons and provide more active sites. As a result, the exfoliated nanosheets (especially the FL-CTF-2 with an apparent quantum yield of 11.14%) show much-improved performance in visible-light-driven HER compared with bulk CTFs. This work provides a new route to tail the photoelectric properties of COFs and may promote their applications in solar energy capture and conversion. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.