Facile design of economic-effective hydrogen evolution reaction (HER) catalysts with non-noble materials are promising for the production of renewable chemical fuels.Two-dimensional (2D) ultrathin transition metal dichalcogenides (TMDs) materials with large specific surface area and abundant catalytic active sites can significantly enhance their catalytic activities.Herein,we design and synthesize an atomically thin Ni-Se-S based hybrid nanosheet (NiSe1.2S0.8) via a simple solvothermal method,the thickness of NiSet.2S0.8 nanosheets is only about 1.1 nm.Benefiting from the ultrathin nanostructure and rich defects,the optimal NiSe1.2s0.8 exhibits good electrocatalytic activity with the overpotential of 144 mV at-10 mA·cm-2,a small Tafel slope of 59 mV·dec-1,and outstanding catalytic stability in acid electrolyte for HER.The theoretical results show that hybrid electrocatalyst by S incorporation possesses the optimal adsorption free energy of hydrogen (△GH·).This study provides a simple method to synthesize a high performance multicomponent electrocatalysts with the ultrathin nanostructures and abundant defects.