Tungsten carbide (WC) with imperfect structures determined by phase engineering and heteroatom doping has attracted a great deal of attention with respect to hydrogen evolution reaction (HER). However, less is known about its surface in HER. Herein, we report a tungsten oxide/carbide surface heterojunction catalyst (SHC) and reveal that the surface heterojunction that oscillates at HER potentials is responsible for high HER activity. This tungsten oxide/carbide SHC is active in both acidic (0.5 M H2SO4) and neutral [0.1 M phosphate buffer (pH 7.02)] electrolytes with a current density of 20 mA cm-2 at 0.32 mg cm-2 at overpotentials of-233 and-292 mV, respectively. From electron paramagnetic resonance spectroscopy and density functional theory calculations, we find that the surface heterojunction relaxed the adsorption of HER intermediates on WC. With in situ X-ray absorption spectroscopy, we are able to relate the HER activity to the bias-stimulated oscillation of the surface oxide/carbide heterojunction, which reflects the strong interfacial electronic coupling. This bias-oscillating surface heterojunction is thus suggested as a unique structural descriptor for WC-based HER catalysts, and the finding could be useful to other SHCs.