The invention provides an ultrasonic plasticizing forming system comprising a mold cavity template, a plasticizing distributing template, a plasticizing assembly and a needle valve assembly. The moldcavity template is provided with a mold cavity. The plasticizing distributing template comprises a body, a plasticizing cavity and at least one distributing channel. The plasticizing assembly comprises an ultrasonic amplitude-change pole and a plasticizing power source. The needle valve assembly comprises at least one needle valve single part comprising a valve needle and a valve needle power source. The application of the ultrasonic plasticizing forming system has the effects that the whole structure is simple; the ultrasonic amplitude-change pole can be inserted into the plasticizing cavity,ultrasonic plasticizing of particles can be achieved, and the particles are completely plasticized into melt; combined design of the plasticizing assembly, the plasticizing cavity in the plasticizingdistributing template and the needle valve single part in the needle valve assembly can be achieved, plasticizing time and plasticizing quality of the particles can be controlled effectively, the effect that a high-quality workpiece is obtained is ensured, raw materials are saved, and cost is lowered. The invention further discloses a forming method particularly adopting the ultrasonic plasticizing forming system to conduct forming, the process is simple, and product quality is good.