Photocatalytic hydrogen evolution is an attractive field for future environment-friendly energy.However,fast recombination of photogenerated charges severely inhibits hydrogen efficiency.Single-atom cocatalysts such as Pt have emerged as an effective method to enhance the photocatalytic activity by introduction of active sites and boosting charge separation with low-coordination environment.Herein,we demonstrated a new strategy to develop a highly active Pd single atom in carbondeficient g-C3N4 with a unique coordination.The single-atom Pd-N3 sites constructed by oil bath heating and photoreduction process were confirmed by HADDF-STEM and XPS measurements.Introduction of single-atom Pd greatly improved the separation and transportation of charge carriers,leading to a longer lifespan for consequent reactions.The obtained singleatom Pd loaded on the carbon-deficient g-C3N4 showed excellent photocatalytic activity in hydrogen production with about 24 and 4 times higher activity than that of g-C3N4 and nano-sized Pd on the same support,respectively.This work provides a new insight on the design of single-atom catalyst.