Abstract: Polylactic acid (PLA) as a biobased degradable plastic has increasingly become a research hotspot. However, PLA's application in packaging and electrical appliances is limited due to its high combustibility. In order to solve the above problem, lignin containing silicon-nitrogen synergistic (Si-NLig) was synthesized by modification of alkali lignin (Lig), and its TGA results showed that the temperature corresponding to mass loss 5wt% of material (T_5%) increased by 20℃ and the high temperature residue increased from 2.3% to 25.5% in air. And Si-NLig was used as a charring agent, and compounded with ammonium polyphosphate (APP) to prepared flame retardant polylactic acid (Si-NLig-APP/PLA) by melt blending and hot-press molding, and the PLAs' flame retardant properties, mechanical properties, and combustion behavior, were investigated. The results showed that addition of 10wt% Si-NLig-APP with the mass ratio 1∶4 made Si-NLig-8%APP/PLA reach the limiting oxygen index (LOI) value of 27% and the vertical burning UL-94 V-0 level, while the LOI value of Lig-8%APP/PLA with Lig as charring agent at the same condition was 26% and the vertical burning UL-94 only passed V-2 rating. Meanwhile, the peak heat release rate (PHRR) reduced by 27% compared to Lig-8%APP/PLA. Raman spectroscopy was used to characterize the structure of the residual char after cone calorimeter testing, it was found that the degree of graphitization of Si-NLig-8%APP/PLA increased by 36.7% compared to that of Lig-8%APP/PLA, which provides a theoretical basis for its good flame retardancy. Introduction of Si-NLig led to the enhancement of mechanical properties of the flame retardant PLA with tensile strength increasing by 21%. It can be seen that Si-NLig has potential application prospect in the field of halogen-free flame retardant PLA.