Abstract: In view of problem of bad control effect because of manual control for medium density control in dense-medium separation in one coal preparation plant, medium density control system based on fuzzy control was designed. The system adds a new PLC and monitoring software to original system. The new PLC reads field data from PLC of the original system and uses fuzzy control algorithm to input control parameters into PLC of original system, so as to control outlet density of qualified medium bucket effectively. The actual application showed that the system has good control effect and simple control method.