Abstract: The cause of emulsion formation of waste water with large qtiantity of naphthenic acid, phenol and sulfur was studied. It is found that the emulsion is formed by repulsive effect of double layer. Cationic surface-active agents can be used to demulsify the emulsion of this kind to increase the oil yield and decrease the pollution. There is yet no such agents being applied to demulsify such oil/water emulsion. A series of new highly efficient demulsifying agents were synthesized; the conditions of preparation were examined and the mechanism of demulsification was investigated. Through demulsifying tests of various kinds of surface-active agents, proper operating conditions of application in oil refinery were also searched.