The invention discloses a preparation method of a nano-iron catalyst for catalyzing the hydrolysis and dehydrogenation of ammonia borane. The method comprises the following specific process: stirringa reducing agent, namely a borane complex, and ferric salt or ferrous salt for a reaction at 10-30 DEG C in the absence of water and oxygen by adopting a one-step chemical reduction method to obtain the nano-iron catalyst, wherein the borane complex is tetrahydrofuran borane THF.BH3 or dimethyl sulfoxide borane DMS.BH3, the ferric salt is ferric chloride, ferric sulfate or ferric nitrate, and theferrous salt is ferrous chloride, ferrous sulfate or ferrous nitrate. The nano-iron catalyst can be repeatedly used after catalyzing the hydrolysis and dehydrogenation of the ammonia borane. The nano-iron catalyst, provided by the invention, has higher catalytic activity than that of a similar iron-based catalyst obtained with a prior method, more effectively promotes the progress of the hydrolysis and dehydrogenation reaction of the ammonia borane, and has excellent cycle stability performance.