Abstract: Polylactic acid (PLA), as a biodegradable material based on biomass, is widely used in many fields. However, due to its own structural and compositional characteristics, PLA is flammable, which limits its practical application in electronics, electrical appliances, automobiles and other fields. Therefore, it is necessary to conduct flame retardant modification research on PLA. Using lignin as the main raw material and dodecyltrimethoxysilane as the modifier, long aliphatic chains and silicon element were introduced into alkali lignin through a one-step process, and the lignin with long aliphatic chains (RLig) was prepared. Thermogravimetric analysis showed that RLig had good charring ability under air condition, and the residual char content at 550℃ increased from 2.3% of the unmodified lignin to 27.8%. RLig was compounded with ammonium polyphosphate (APP) to prepare flame retardant polylactic acid composite materials (RLig-APP/PLA), and the flame retardant properties, combustion behavior, and mechanical properties were researched. The results indicated that when the mass ratio of APP to RLig was 4∶1, a total addition of 10wt% made 2RLig-8APP/PLA have the limiting oxygen index(LOI) value of 27%, and pass the vertical combustion UL-94 V-0 grade. While the LOI value of 2Lig-8APP/PLA with unmodified Lig was 26% and the vertical burning UL-94 only passed V-2 rating. Meanwhile, the analytic results of the cone calorimeter test showed that the peak heat release rate (pHRR) of 2RLig-8APP/PLA was significantly reduced, and the fire safety was greatly improved. The analysis of condensed and gas phase products showed that the introduction of RLig made the flame retardant mechanism of flame retardant PLA dominated by gas phase flame retardant, supplemented by condensed phase flame retardant. In addition, the mechanical properties of 2RLig-8APP/PLA were significantly improved, with a tensile strength of 58.3 MPa and an impact strength of 9.6 kJ·m⁻², more than doubled compared to 2Lig-8APP/PLA. At the same time, the storage modulus of 2RLig-8APP/PLA were also improved.