The invention discloses a preparation method of a molybdenum dioxide nanowire/nanosheet thin film array with excellent electrocatalytic hydrogen evolution performance. The preparation method comprisesthe following steps: weighing 0.6-0.8 g of ammonium molybdate in a beaker, then adding 0.2-0.3 ml of 30 percent of hydrogen peroxide and 0.1-0.3 ml of absolute ethyl alcohol, adding deionized water to 30 ml, stirring till full dissolution, then putting a solution in a polytetrafluoroethylene inner container, putting a 2-4-cm molybdenum mesh in the polytetrafluoroethylene inner container, puttingthe polytetrafluoroethylene inner container in a metal reaction kettle, tightening, putting in a hydrothermal baking oven for hydrothermal reaction, performing heat preservation for 6-15 hours at a temperature of 150-230 DEG C, finally washing a sample for several times with deionized water, performing heat preservation for 1 hour at a temperature of 60 DEG C and drying to obtain the molybdenum dioxide nanowire/nanosheet thin film array with a black appearance. The product synthesized with the preparation method has relatively large specific surface area and more active sites, and overcomes the defect of poor oxidation conductivity.