A thorough analysis of the mechanism of vibrational force field on polymer processing was carried out to investigate the effect of vibration parameters on polymer viscoelasticity and plastic forming processing. The relationship between phase angle and lagging heat generation rate as well as the expression of polymer plasticizing rate under vibrational force field was obtained. To verify the theory, PET was adopted for example calculation and in expriments with dynamic capillary rheometer and mlti-dimentional vibration induced plastic forming equimpment. The result from calculation agrees well with that from experiment. Besides, it can be concluded that the plasticizing rate of PET increases at first and decreases afterwards with increasing vibration frequency. It reaches the peak at vibration frequency equivalent to the natural frequency of PET of about 15 Hz. The value remains at 22 g/min with the natural vibration frequency when the vibration amplitude is 6 MPa, being 1.75 fold that with vibration frequency of 10 Hz. Therefore, the plasticizing rate of polymer can be significantly improved by increasing the vibration amplitude. The exploration in this paper supplies reliable thoeretical reference for novel polymer low temperature moulding technique and equipments.