Abstract: About 15000t of oily scum with high water-containing can be produced per year by Yangtze Petrochemical Refinery.Experiments of using ultrasound pre-processing to reduce water-containing,as well as reduce the volume of the oily scum have been made.Results give the optimum values of some factors influencing dehydration effect,such as frequency,intensity,lasting time of ultrasound,sedimentation time and temperature.Comparing with the original technology of adding flocculants into the scum,the ultra-sound method is obviously superior in saving the depletion of flocculants to make the processing cost much lower for the same effect,and meantime in making the water-containing of the oily scum lower under the same condition of sedimentation.