The invention discloses a preparation method of metal nanosheets. The preparation method comprises the steps that 1, strong ammonia water is added into a nickel chloride solution, a NaOH solution is added, a hydrazine hydrate solution is added, the solutions are stirred and mixed to be uniform, and a mixed solution is obtained; 2, a substrate can be placed in a lining of a hydrothermal reaction kettle, the mixed solution is transferred into the lining of the hydrothermal reaction kettle, the hydrothermal reaction kettle is sealed, the constant temperature ranging from 80 DEG C to 120 DEG C is kept for 12-24 hours, and crude nickel nanosheets or crude nickel nanosheets growing on the substrate are obtained; 3, the crude nickel nanosheets are repeatedly cleaned multiple times in pure water and ethyl alcohol, drying is conducted, and the nickel nanosheets are obtained. According to the preparation method, the nickel metal nanosheets are obtained through spontaneous growth by means of a mild organic surfactant-free method, the thickness of the nickel metal nanosheets can be controlled to be 10 nanometers or below, the prepared nickel nanosheets are high in quality, the surfaces of the nickel nanosheets are not covered with active agents, and the nickel nanosheets are low in cost and suitable for mass production.