The thermal degradation of polylactide (PLA) during melt processing may lead to poor product properties. In this study, a unique Raman system was applied to explore the thermal degradation kinetics of molten PLA under five different temperatures and three atmospheres. It was found that the intensity of specific Raman bands correlated with the concentration of functional groups in the PLA during the degradation process. Kinetic models for thermal degradation were established based on the intensity of the characteristic band of C-COO. Kinetic parameters determined by kinetic models quantitatively revealed the effect of temperature and atmosphere on thermal degradation. Furthermore, time sweep rheological tests using the same degradation conditions were performed for verification, and the results corroborated the spectral analyses. This investigation demonstrates the feasibility of Raman spectroscopy for thermal degradation kinetic analysis.