The invention belongs to the technical field of chemical synthesis, and particularly relates to a preparation method of hydrazine hydrate. The preparation method comprises the following step of hydrolyzing ketazine in a distillation tower to obtain the hydrazine hydrate, and is characterized in that a proper amount of catalyst is added into hydrolyte for preparing the hydrazine hydrate by hydrolyzing the ketazine; the catalyst is one or a mixture of two of hydrazine hydrochloride and hydrazine sulfate. The preparation method has the characteristics that the hydrolyzing reaction is effectively promoted; the hydrolyzing reaction of the ketazine is performed at normal pressure; the temperature and pressure of hydrolyzing reaction are reduced, the hydrolyzing time is shortened, the yield rate of the hydrazine hydrate is increased, the steam consumption is reduced, the requirement on the hydrolyzing device is low, the risk in the hydrolyzing process is little, the industrialization is simple, the economic benefit is obvious, and the like.