The invention discloses a method for preparing monoammonium phosphate and co-producing a special compound fertilizer for potatoes. The method comprises the following steps: preparing raw materials; preparing monoammonium phosphate; adding filtrate elements; and drying. The special compound fertilizer suitable for potatoes is processed by using mother liquor aiming at content of elements required by potatoes and research and analysis of absorptivity of potatoes on elements as well as improvement and adjustment of a process for preparing monoammonium phosphate, so that the waste on resources is reduced, the cost is lowered and the economic benefit is improved.