The work aims to study the thermal decomposition kinetics of injection grade polylactic acid (PLA) and accurately understand the heat stability of injection grade polylactic acid (PLA), so as to provide theoretical basis for developing high temperature and flame retardant injection grade polylactic acid (PLA) materials. By non isothermal weight analysis, the heating rate of 5, 10, 15, 20 and 25 ℃/min was adopted to analyze the thermal decomposition behavior of injection grade PLA in nitrogen atmosphere. One differential method and three integral methods were used to carry out the detailed kinetics calculation. The correlation coefficient and standard deviation were compared. KAS isothermal integration method was selected, and with relative deviation Ai=|1–αc/αe| as the objective function, 16 kinds of thermal decomposition kinetics mechanisms were used to simulate and calculate the most appropriate reaction mechanism for the thermal decomposition of injection grade PLA. The activation energy and pre-exponential factors required for thermal decomposition of injection grade PLA were obtained. The activation energies of injection grade PLA calculated by Kissinger, Madhusudanan-Krishnan-Ninan (MKN), Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) were 177.01, 174.43, 173.01 and 173.28 kJ/mol, respectively. The pre-exponential factors were 25.84, 26.69-33.75, 25.83-32.89 and 26.38-32.94, respectively. The random nucleation and subsequent growth reaction mechanism (A1/4) are determined and ln(β/T2)=ln[4.75×109/–ln(1–α)]–2.08×104/T is the most appropriate description of injection grade PLA thermal decomposition reaction mechanism.