Hydrogen evolution reaction (HER) activities could be enhanced on MoS₂ through exotic doping by introducing active centers. In this work, we explored HER activity on MoS₂ edge by alloying isovalent W into MoS₂ nanosheets. The first-principles calculations predicted that optimum HER performance may be achieved with W substituting 50% of Mo in MoS₂ due to the decrease of H adsorption free energy ΔG_H and facilitation of charge transfer at the electrode interface. At a H coverage of 25% on Mo_0.5W_0.5S₂, the value of ΔG_H reaches minimum which favors both H adsorption and desorption processes on catalyst surface, and electronic conductivity of the material is also enhanced through PDOS calculation. Experiments demonstrated that Mo_0.5W_0.5S₂ significantly enhanced HER performance with a low onset potential of -37 mV, a Tafel slope of 55 mV dec^-1, and an overpotential of 138 mV at 10 mA cm^-2, which is among the best performance of reported MoS₂ based compounds. The Mo_0.5W_0.5S₂ also showed excellent stability during polarization.