The work aims to study the effect of sodium citrate on the electrodeposition mechanism of Ni-Sn-Mo alloy in an acidic sodium citrate system, and to find out the maximum electrode reaction rate and the corresponding sodium citrate concentration of the alloy coating with the best hydrogen evolution performance. The electrodeposition mechanism and hydrogen evolution behavior of Ni-Sn-Mo alloy with different sodium citrate concentration were studied by cyclic voltammetry curve, electrochemical impedance spectroscopy, chronoamperometry and cathodic polarization curve. The surface morphology of Ni-Sn-Mo alloy was characterized by scanning electron microscopy and its composition was detected by energy spectrometer. The electrodeposition of Ni-Sn-Mo alloy obtained by cyclic voltammetry curve was an irreversible process. When the corresponding sodium citrate concentration was 0.2 mol/L, the potential of the alloy co-deposition reduction peak was the most positive (-0.96 V(vs. Ag/Ag+)). According to the electrochemical impedance spectroscopy, when the concentration of sodium citrate was 0.1 mol/L, diffusion characteristics appeared at the low frequency end, and when sodium citrate concentration was 0.2~0.4 mol/L, the reaction was not affected by diffusion. When the concentration of sodium citrate was 0.2 mol/L, the polarization resistance (Rp) reached the minimum value (11 718.1 Ω?m2) and electrode reaction was most likely to happen. Through CA curves, the crystal nucleation of Ni-Sn-Mo alloy always followed the rule of continuous nucleation as the concentration of sodium citrate increased. SEM and EDS results showed that the grain of Ni-Sn-Mo alloy decreased and then increased with increase of sodium citrate concentration. With the increase of Sn content in the alloy, the grain of the alloy changed from cellular to irregular shape. According to the cathodic polarization curve and electrochemical impedance spectrum, when the concentration of sodium citrate was 0.1 mol/L, the hydrogen evolution potential (-1.05 V) was the most positive, the charge transfer resistance (4.906 Ω?cm2) was the smallest, and the hydrogen evolution performance was the best, which improved the hydrogen evolution performance of the alloy from the aspects of energy and geometry. When the concentration of sodium citrate is 0.2 mol/L, the reduction peak potential is the most positive and the electrode reaction is most likely to happen. Adding Sn to Ni-Mo alloy coating increases the specific surface area of coating and improves the electron transfer rate, which is conducive to the improvement of hydrogen evolution performance.