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Molybdenum carbides show great potential to replace platinum for electrocatalytic hydrogen evolution reaction (HER) to resolve the problem of hydrogen production, due to their high reserves, stability, low cost, and structural diversity. However, the effect of atomic configurations of different surfaces on HER is still lacking theoretical insights. In this work, the HER activity on 29 surfaces of seven phases is systematically explored by density functional theory, taking into account water effect. The exchange current for each surface is also given. Totally, there are nine surfaces which own high exchange current (>0.1 mA/cm2), especially the hydroxylated (014)-C and (010) of TiP-MoC, (110) of β-Mo2C, and (100)-C of α-Mo2C (1.410, 0.835, 0.687, and 0.464 mA/cm2, respectively). Combining with the stabilities of the surfaces for each phase, the phases with high HER activity could be also screened out. The electronic properties, including electron transfer to adsorbed hydrogen and the shift of the electronic states coupled by oxygen and adsorbed hydrogen orbitals, are applied to uncover the termination of surface and water effect on HER. Our results are expected to contribute to the understanding of the HER on different surfaces of molybdenum carbides and give some evidence for control synthesis of high HER activity surfaces.