A method of controllably preparing ferric oxide powder with cattail wool relates to the technical field of preparation of ferric oxide powder. The method includes the steps of: forming a reaction system with an alkaline extract liquid of cattail wool and ferric chloride and controlling the pH of the reaction system to 4-14 by changing addition amount of the alkaline extract liquid; and performing a hydrothermal reaction and post-treatment to prepare the ferric oxide powder. In the method, two kinds of ferric oxide powders are successfully prepared through a simple hydrothermal method. A series of representation shows that alpha-Fe2O3 and Fe3O4 powders prepared from the reaction system formed by the alkaline extract liquid of cattail wool have narrow particle size distribution and uniform particle size, are high in purity and have strong ferromagnetism. The method can utilize the abundant cattail wool resource and is low in preparation cost. The constitution and appearance of the products are controllable. The method has simple operations and is hopeful to be applied to large-scale production of high-quality ferric oxide powders.