Abstract: Following an introduction to the structural design,working principle and usage of the ORCA sonar interface meter,the paper elaborates on the functions of the meter in the coal preparation plant for guiding the automatic addition of flocculants in thickener,start-up of underflow pump and monitoring of thickness of supernatant layer,as well as the result of practical application of the meter at Yanjing plant.Practice shows that the use of the meter can lead to effective reduction of flocculants and power consumption and operating cost,and facilitated management of coal slurry water treatment process.