To solve the poor conductivity and limited active sites of MoSe₂, ternary MoS_xSe_2-x nanosheets with defects were synthesized by a convenient hydrothermal process. The results showed that the introduction of S element not only improved the electron transfer ability, but also provided more electrocatalytic active sites. Consequently, the optimized MoS_xSe_2-x with a S/Se molar ratio of 1:1 (MoSSe) presented superior electrocatalytic hydrogen evolution reaction performance with a low Tafel slope of 47 mV/dec, and a small overpotential of -165 mV at -10 mA/cm² as well as good durability. This work provides an additional route for better understanding the modulation of multi-factors in designing and synthesizing MoSe₂-based catalysts to improve their electrochemical activity.