The analysis of the ultrasonic plastic welding process is conducted in this paper.It indicates that the pressure has an evidently different function in each time phase of the whole welding process consisting of initial melting,spreading of the melt,formation of the melt layer,squeezing out of the melt and the solidation of the joint.Consequently it is difficult to optimize the welding pressure in pressure constant mode used in conventional ultrasonic plastic welder.AVEP312 type pressure control proportional valve is used in the ultrasonic plastic welder with the energy control mode realizing the pressure adjustment during the welding process,hence laying the fundamental for better weld quality control.