Abstract: The ultrafine coal slime is taken as the object to explore the effects that can be obtained in dewatering of the material through addition of filter aid and ultrasonic,assisted treatment.Analysis is made of the property of the slime and the combined influences on the effect of slime dewatering produced by such factors as dosage of filter aid, ultrasonic frequency and ultrasonic action time, among others.Results of test on dewatering of the slime of Hecaogou Coal Preparation Plant show that under the combined actions of filter aid and ultrasonic wave, and with the use of calcium chloride, polyaluminum chloride and polyacrylamide (PAM) respectively dosed at 1,600g/t, 1,000g/t and 70g/t, and a ultrasonic treatment time of 7 minutes, the moisture of the dewatered slime is two percentages lower than that of the untreated slime, and the filter dewatering time is cut by 45 minutes.It is found that ultrasonic treatment can lead to effective reduction of dewatering time and further cutdown of moisture of filter cake.But an excessively long ultrasonic treatment time has contrarily an adverse effect on the improvement of dewatering efficiency because of the presence of new ions on coal particle surface generated under the ultrasonic cavitation and mechanochemical effect.