The decomposition and formation of water molecules determine the protonation and deprotonation processes at the catalyst surface. Detecting interfacial water is particularly challenging because it is located between two condensed phases. Herein, we combine electrochemical in situ Raman and infrared spectroscopy with computational techniques to study the interfacial water on snowflake-like 1 T/2H-xMoS2/Cu2S (1 T/2H-MxCS). The snowflake-like Cu2S prevents the aggregation of modified MoS2 nanoflowers. The 1 T/2H-M0.02CS electrocatalyst exhibits excellence electrochemical performance, with a low overpotential of 59 mV and Tafel slope of 56 mV dec−1. This outstanding property can be ascribed to the interface and the formation of 1 T-MoS2, which not only decreases the activation energy required for the reduction reaction but also provides additional active sites for the catalytic reaction. Density functional theory calculations (DFT), in-situ Raman, and FT-IR spectroscopy were used to further elucidate the mechanism of electrocatalytic performance. This strategy offers new insights into enhancing the electrocatalytic activity of transition metal sulfides and a deep understanding of the HER mechanism.