The polyethylene terephthalate (PET) structure was studied by IR spectrum (IR spectrum). The PET CH2 bending vibration mode (δCH2) included CH2 gauche bending vibration mode (δCH2-gauche) and CH2 trans bending vibration mode (δCH2-trans). The temperature-dependent IR spectrum (TD-IR spectrum) of PET/δCH2-gauche and PET/δCH2-trans were researched also. It was found that with the increase of the measurement temperature, the IR absorption frequency and intensity of PET/δCH2-gauche and PET/δCH2-trans were changed obviously and the glass transition temperature (Tg) was a critical temperature Trans and gauche conformation transition mechanisms of PET were further researched by the two dimensional IR spectrum (2D-IR spectrum). The study expands the research scope of three-step IR spectrum (including: IR spectrum, TD-IR spectrum and 2D-IR spectrum) in the anal. of structure and conformation transition of the important polymer material (PET).