Combining the advantages of multiple strategies including morphology control, atom doping, and defect construction, the photocatalytic hydrogen evolution reaction (HER) performance of graphitic carbon nitride (g-C3N4) may be boosted significantly. Herein, holey ultra-thin g-C3N4 nanosheets (CNS-x, x = H, O, N) modified with structural vacancies and sulfur (S) dopant are synthesized through a straightforward method involving the pyrolysis of thiocyanuric acid under various gas atmospheres. The textural properties, type and concentration of structural defect, and S doping level are significantly influenced by the gas atmosphere. Remarkably, CNS-H with C, N dual defects and S-dopant exhibits the HER rate of 46.59 mmol·g−1·h−1, which is 16.68 times higher than that of bulk g-C3N4. The corresponding apparent quantum yields are 17.24 % at 420 nm and 1.90 % at 520 nm. The enhanced HER performance of CNS-H can be attributed to the combined effect of improved charge separation, extended photoabsorption, and optimized properties of the platinum cocatalyst.