Doping engineering has been considered as a viable strategy to obtain highly efficient photocatalysts for hydrogen evolution reaction (HER). Since transition metal doping usually introduces of oxygen vacancy defect in oxide compounds, the first-principles calculations were performed to investigate the HER activity of transition metal-doped lepidocrocite-type TiO2 nanosheets with oxygen vacancies. Different hydrogen adsorption sites are taken into account here. When adsorbed hydrogen occupies the oxygen vacancy, an optimal HER activity, similar to ideal Pt metal, was obtained in Fe-doped nanosheets. The tunable HER performance was correlated with the d-band center level of dopants. On the other hand, O-p(z) band center level is responsible for the hydrogen adsorption free energy in the case of hydrogen adsorbed with the surface oxygen atom. Unexpectedly, for Mn doping cases, the pre-adsorbed hydrogen could activate surface O atom with additional hydrogen adsorption free energy being close to zero. In addition, the strain engineering also could effectively adjust HER activity in defected nanosheets. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.