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During the exploration of highly efficient noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER), a promising and challenging strategy is to fabricate composite nanocatalysts by finely tuning metal and/or nonmetal element components. Herein, we report a new HER electrocatalyst, which is composed of molybdenum phosphide and molybdenum carbide composite nanoparticles (NPs) coated by few-layer N-doped graphitic carbon shells (denoted as MoP/Mo 2 C@C). Such a new combination mode of electrocatalysts is realized by a one-step annealing route with the mixture of a Mo/P-based polyoxometalate (POM) and dicyandiamide. On the basis of this method, the simultaneous phosphorization and carbonization in a nanoscale confined space can be easily achieved by the use of POM as the molecular-element-regulating platform. MoP/Mo 2 C@C exhibits more remarkable HER performance over the whole pH range than those of MoP, Mo 2 C, and the physical mixture of MoP and Mo 2 C. The low overpotentials of 89, 136, and 75 mV were obtained at a current density of 10 mA cm -2 in the media of pH = 0, 7, and 14, respectively. Furthermore, MoP/Mo 2 C@C shows a long-term durability for 14 h over the entire pH range (0-14). Because of the protection of carbon shells, such composite electrocatalyst also possesses better transition-metal tolerance exemplified by Fe 2+ , Co 2+ , and Ni 2+ than that of 20% commercial Pt/C. This work demonstrates the advantage of POM precursors in adjusting the component and properties of nanoscale composite electrocatalysts for HER, which may suggest new options for the fabrication of highly efficient composite electrocatalysts.