Abstract: The self-made lignin-based charring agent with phosphorus (Lig-P) and ammonium polyphosphate (APP) were used for the preparation of flame retardant poly(lactic acid) (PLA), and organic montmorillonite (OMMT) was also incorporated into flame retardant PLA, and the synergistic effect of OMMT on the flame retardant properties of PLA was studied in detail. The flame retardancy, thermal degradation and fire performance of the Lig-P-APP-OMMT/PLA composites were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimetric test and TGA, respectively. The analysis results show that there is significantly synergistic effect between OMMT and Lig-P-APP for Lig-P-APP-OMMT/PLA composites, in which the LOI value of PLA increase from 27% to 32%, accompanied with the PLA passing V-0 rating in UL-94 test at 3wt% of OMMT substitution for Lig-P-APP. The peak thermal degradation rate of Lig-P-APP-OMMT/PLA flame retardant composites reduces, and its charring residue at 800℃ increases by nearly 50%. Cone calorimetry results indicate that the heat release rate of Lig-P-APP-OMMT/PLA flame retardant composites dramatically reduces, and the peak heat release rate (PHRR), the peak smoke produce rate (PSPR) and the total smoke production (TSP) decrease by 26.4%, 60% and 26.3%, respectively with the optimum loading of 3wt% OMMT. The incorporation of OMMT benefits to the formation of much more compact and more graphitic char layer during the combustion of Lig-P-APP-OMMT/PLA flame retardant composites.