Abstract: In order to improve the water purification effect and automation management level,Tunlan coal preparation plant introduced SDM4000 ultrasonic densometer. The paper introduced the structure,working principle and technical parameters of the densometer,and elaborated its installation mode and operation essentials. The successful use of SDM4000 ultrasonic densometer ensured the washing water purifying effect,and saves reagent dosage,which has a great significance to improve water quality.