The invention discloses a method using aluminum ash to prepare a water purifying agent. The method comprises the following steps of sending the aluminum ash into a washing layer through an aluminum ash inlet, and closing the aluminum ash inlet; sending boiled water into the washing layer through a boiled water inlet, and cleaning the aluminum ash in the washing layer; after cleaning, opening a second closable passage, sending the aluminum ash and wastewater into a first filtering layer, enabling the wastewater to flow out of a first liquid outlet, remaining the cleaned aluminum ash onto the first filtering layer, opening the first closable passage, transferring the first filtering layer into a drying device, closing the first closable passage, and using a first heating device at the bottom part of the drying device to dry the aluminum ash on the first filtering layer; after drying, using an absorbing device to convey the aluminum ash onto a weighing device; using the weighing device to weigh the aluminum ash, and sending the aluminum ash into a reaction layer of the reaction kettle through a material inlet, so as to solve the problems of low utilization rate of aluminum ash and pollution to environment.