The invention relates to a method for preparing amorphous FeOOH, which comprises the following steps: (a) mixing solid soluble ferrous salt and solid hydroxid according to a molar ratio of iron to hydroxyl being between 1:2 and 1:3; (b) completing reaction of a mixed material prepared in step (a) by kneading, and controlling the temperature of the material not to exceed 70 DEG C in the kneading process; (c) drying a reaction product obtained in step (b) in air; (d) washing and filtering a material obtained in step (c); and (e) naturally drying or baking a solid obtained in step (d). On the basis, the invention further discloses an amorphous FeOOH desulfurizer prepared by the preparation method, and the desulfurizer has high sulfur capacity and high desulfurization efficiency.