Cost-effective electrocatalysts for the hydrogen evolution reaction (HER) play an important role in the field of renewable energy. Although enormous efforts have been spent on searching for alternative materials, Pt is still the most efficient HER electrocatalyst. Therefore, minimizing the loading amount of Pt in the electrocatalysts is significant. Herein, a new catalyst, uniformly dispersing Pt clusters on MoO2 nanosheets via oxygen vacancy anchoring Pt clusters on MoO2 NSs by a room-temperature light-reduction method (Pt Cs/MoO2 NSs-L), exhibits superior HER activity. The area current density of 0.5 wt% Pt Cs/MoO2 NSs-L (12.2 mA/cm2) is close to that of 20 wt% Pt/C (20 mA/cm2), but corresponds to a mass activity (7.43 A/mg) of 0.5 wt% Pt Cs/MoO2 NSs-L, 26.5-fold greater than that of the state-of-the-art commercial 20 wt% Pt/C (0.28 A/mg) at an overpotential of 50 mV vs. RHE. Density function theory (DFT) calculations confirm that the adsorption/desorption performance of H∗ is significantly improved on Pt and nearby Mo which the Pt is anchoring on the oxygen vacancies of MoO2. The present work demonstrates the feasibility of fabrication efficient HER electrocatalysts via a room-temperature light-reduction method and assistance of reducing with oxygen defects.