Abstract: The preparation of efficient catalysts in hydrogen evolution reaction (HER) is an urgent task at present. In this work, Ni(OH)₂/Ni/g-C₃N₄ composite catalyst was prepared through liquid phase impregnation with in-situ reduction, which was used to compose the cathode with carbon paper (CP) for the microbial electrolysis cell (MEC). With the help of SEM, TEM, XRD, XPS and electrochemical analysis techniques, the structure, properties and electrocatalytic performance in hydrogen evolution of the Ni(OH)₂/Ni/g-C₃N₄ composite were investigated. The results indicate that the Ni(OH)₂/Ni/g-C₃N₄ catalyst exhibits excellent electrochemical activity for hydrogen evolution in the MEC. Using the Ni(OH)₂/Ni/g-C₃N₄ catalyst, the current density reaches 100 A/cm² at a small overpotential of 1881 mV, with a low charge transfer resistance of 10.86 Ω and a low Tafel slope of 44.3 mV/dec, which is much superior to pure g-C₃N₄ catalyst and CP, and even comparable to the Pt catalyst, suggesting that the Ni(OH)₂/Ni/g-C₃N₄ composite can be a potential candidate of HER catalyst in MEC.