The invention belongs to a preparing device and method of compound fertilizer mixed slurry. The preparing device comprises a compound fertilizer production raw material producing and storing device which is connected with a granulating device through a mixing, heating and stirring device, a pump, a first feeding solid-liquid mixing device, a second feeding solid-liquid mixing device and a tubular solid-liquid mixing device in sequence. The preparing device is simple in structure and reasonable in process design, reducing investment cost and occupation area and obtaining compound fertilizer good in performance while energy consumption is reduced.