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The intrinsically phosphorous-containing flame-retardant poly (lactic acid) (P-PLA) was synthesized through the chain extension of L-lactide and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivative, then P-PLA was melt blended with neat PLA to produce flame-retardant PLA composite (FR-PLA). The flame-retardant properties of FR-PLAs were investigated by Limiting Oxygen Index (LOI), UL-94, and Cone Calorimeter Tests, the results indicated that the flame-retardant properties of PLA composites were significantly improved with the addition of P-PLA due to its excellent ability to carbonize and dilute combustible gases. The LOI value of FRPLA10 composite (consisting of 90 wt％ neat PLA and 10 wt％ P-PLA) increased from 20.5 to 27.8 and UL-94 rating reached up to V-1, the peak heat release rate and total heat release of FR-PLA composite were lower than those of neat PLA. The flame-retardant mechanism of FR-PLA depended on both condensed and gaseous phases, which was confirmed by the FTIR, Raman and TG-FTIR. Besides, the tensile strength of FR-PLA10 slightly decreased from 72.4 to 61.8 MPa, which met the requirements of most practical applications. Therefore, the modification method used in this work presented a feasible approach to prepare FR-PLA composite with satisfactory mechanical property.