Abstract: In order to tackle the problem encountered in density control of the small coal rewashing system, work is made on optimization of the heavy-medium suspension density control system through the following procedures: the first step is to improve the existing coal washing process flowsheet and use proportional valves in place of the currently used flow splitting and make-up water valves, so as to lay a foundation for enabling the density control system to realize PID fine control; and the next step is to optimize the automatic density control system. The medium suspension density control algorithm based on expert experience is proposed. The flow diversion valve is under the control of the expert system. Through flow diversion control, the suspension density can first be made close to the given value, which is then precisely adjusted by the PID control algorithm through fine adjustment of the opening of the make-up water valve. The two closed-loop systems work independently yet in a mutually restrictive and complementary basis; which can eliminate the phenomena of overshooting or oscillation as encountered in the use of traditional PID multivariable control.