Interfacial microenvironment is critical for the effective matching of catalyst and electrolyte. In this work, interfacial microenvironment is modulated to simultaneously improve catalytic activity and durability of Co3Mo. The dissolution of Mo atoms in Co3Mo during HER produces MoO42-, which can be further dimerized into Mo2O72- at high overpotential. Adsorption of Mo2O72- at the catalyst interface modulates the electronic structure of the surface active atoms, thereby reducing the Gibbs free energy required for HER. An optimal ratio of Mo2O72- with a critical MoO42- concentration of ~13.4mM in the electrolyte can make a positive contribution to catalytic activity, which is evidenced by ultralow overpotential of 78mV at current density of 100mA·cm-2. Meanwhile, almost no voltage increase is detected after 325hours at a current density of 0.5A·cm-2 in an anion-exchange membrane electrolyzer. This investigation provides a strategic and scientific route for industrial electrolyzer based on Mo-based alloys.