The invention discloses a monocrystal porous V2Se9 nanosheet array electro-catalysis material. A V2Se9 nanosheet array grows on a conductive substrate, and V2Se9 is of a monocrystal porous structure. A preparation method of the monocrystal porous V2Se9 nanosheet array electro-catalysis material comprises the following steps: preparing ammonium metavanadate solution with the concentration of 1-4 mol / L, adjusting pH to be 1-3, transferring the solution to a reaction still, putting a conductive substrate in the reaction still, sealing the reaction still, carrying out first-time hydrothermal reaction, growing an ammonium metavanadate nanosheet array on the surface of the conductive substrate, taking out the conductive substrate, washing the surface of the conductive substrate with deionized water, and drying the conductive substrate; calcining the treated conductive substrate in the first step in a muffle furnace to obtain a porous V2O5 nanosheet array; rapidly transferring prepared sodium hydrogen selenide solution to the reaction still, obliquely placing the obtained conductive substrate in the reaction still, sealing the reaction still, then, putting the reaction still in a blasting drying oven, and carrying out second-time hydrothermal reaction; and preparing the V2Se9 nanosheet array. The monocrystal porous V2Se9 nanosheet array electro-catalysis material reduces the hydrogen production cost and has the catalytic activity equivalent to that of platinum.