Using carbon dioxide (CO2) as a physical-blowing agent to prepare microcellular polymeric foams has attracted much attention recently. The foaming process of a semi-crystalline polymer is much more complex than that of an amorphous polymer. The investigation of crystallization behavior of semi-crystalline polymer in CO2 is of great significance to control the foam morphology. In this paper, the conformational changes and crystallization dynamics of poly(L-lactide) (PLLA) during cold-crystallization process under CO2 are studied by an in situ high pressure FTIR. FTIR results show that there is a small-wavenumber shift and intensity increase for some characteristic absorbance bands during the isothermal cold crystallization process of PLLA due to the intermolecular interaction of PLLA chains with CO2. Under CO2 conditions, there are some new bands appearing investigated by difference spectra, which are possibly produced by the specific interaction of CO2 with PLLA. Moreover, the cold crystallization kinetics of PLLA under atmosphere and CO2 have been investigated by analyzing difference spectra. At low crystallization temperature, it has been found that CO2 accelerated the crystallization process of PLLA because of the plasticization of CO2 which decreased the Tg of PLLA. The in situ FITR studies of the interaction of CO2 with PLLA and crystallization dynamics of PLLA under CO2 has been utilized to develop various unique foam morphologies of PLLA.