The invention relates to a method for reducing cellulose crystallinity, aiming at solving the problem that an existing lignocellulose raw material produced fuel ethanol hydrothermal pretreatment process is not conducive to a subsequent fermentation process due to the increase of cellulose crystallinity after pretreatment. The method comprises the following steps: I. mixing cellulose with a Fe<3+> solution and uniformly stirring so as to obtain a mixture; II. adding the mixture into the liner of a hydrothermal reaction kettle, uniformly stirring in N2 atmosphere under a certain pressure and rotating speed, warming up the reaction kettle, preserving the heat in N2 atmosphere under a certain pressure and rotating speed, and finally cooling the reaction kettle to room temperature in cold water; and III. opening the reaction kettle so as to obtain a solid-liquid mixture, filtering the solid-liquid mixture so as to obtain a crude solid product, flushing the crude product and drying, thus achieving the method for reducing cellulose crystallinity. The method disclosed by the invention can be used for reducing cellulose crystallinity.