Developing noble metal-free, inexpensive, and highly active cocatalysts to increase the photocatalytic activity of photocatalysts and promote the practical application is significantly important. In this work, ultrafine carbon-deficient tungsten carbide (WC_1–x) nanocrystals with an average size of 1.98 ± 0.29 nm are successfully prepared as cocatalysts to dramatically enhance the photocatalytic activity of graphitic carbon nitride (g-C₃N₄). The optimized system (WC_1–xCN5) exhibits the best photocatalytic H₂ production rate of 124.5 μmol h^–1 (2490 μmol h^–1 g^–1), which is about 56 times that of bare g-C₃N₄. In this system, ultrafine WC_1–x nanocrystals play a multifunctional role: effectively boosting the carrier separation and transfer and providing rich active sites for H₂ production. Hence, the loading of WC_1–x nanocrystals remarkably increases the photocatalytic H₂ production activity of g-C₃N₄. This work demonstrates that ultrafine WC_1–x nanocrystals have practical application potential to enhance photocatalytic H₂ evolution of g-C₃N₄.