Abstract: The PET (Polyethylene terephthalate) as the object of study,in supercritical carbon dioxide environment,the influence of system pressure and temperature on the raw material physical properties was studied.And samples glass transition temperature (tg ),melting point (tm ),crystallinity,characteristic viscosity (),molecular structure were characterized by TA Universal Analysal 2000(DSC),automatic viscosity locator,FT-IR.The results showed that when the supercritical temperature is 130 ℃ and supercritical pressure is 20 MPa,samples’ tg,tm, have minimum value.At the same time,the crystallinity of samples the lowest,which is droped from 28% to 23% compared with raw material.So there is more CO2 could diffuse into PET,it is to the benefit of foam PET.FT-IR characterization result showed sample molecular structure did not change,that supercritical CO2 and PET haven’t chemical reaction,physical parameters change is due to CO2 diffuse into molecules chain caused the change of crystallinity change.