The invention relates to an electric treating desalting tank of compound ultrasonic wave demulsification. An outer cylinder body sleeved with an insulation sleeve is connected with flange cover plates or dished heads on two sides by using a round cylinder body to form a closed horizontal tank body, an emulsified crude oil feeding pipe is connected to the lower middle part of one side of the outer cylinder body, an oil discharging pipe is connected to the top of the outer cylinder body and is positioned on the other side of the outer cylinder body and at the tail end of the round cylinder body; a drain pipe is connected to the bottom of the outer cylinder body and is positioned on the same side with the oil discharging pipe and at the tail end of the round cylinder body; an ultrasonic amplitude transformer is positioned on the horizontal center axial line of the tank body and is a cylindrical metal body, and the horizontal length of the ultrasonic amplitude transformer is between the horizontal lengths of the emulsified crude oil feeding pipe and the oil discharging pipe or drain pipe; and an electrode net is arranged above the horizontal center axial line of the tank body and comprises an upper-layer metal net and a lower-layer metal net, the metal nets are horizontally arranged square grids or rectangular grids or vertical crossed metal strip nets, the spacing between the two layers of metal nets is between 1cm and 50cm, the distance between the metal nets and the ultrasonic amplitude transformer is not less than 1cm, and the dewatering rate of the electric treating desalting tank is 94-95.0 percent.