A novel phosphaphenanthrene derivative, MAH-DOPS was synthesized with DOPS and maleic anhydride (MAH), and it was added into polylactic acid(PLA) to form a series of PLA/MAH-DOPS composites by melt blending. The structure of MAH-DOPS was characterized by Fourier infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) and high resolution mass spectrometry (HR MS). The thermal stabilities, flame-retardant, mechanical properties and thermal degradation behaviour of MAH-DOPS on PLA are investigated by TG, LOI, UL-94, CONE, TG-FTIR and SEM-EDS. The results show that the P and S elements in the flame retardant MAH-DOPS exert a synergistic flame retardant effect, increasing the flame retardant efficiency and improving the flame retardant properties of PLA. At flame-retardant content of 5 wt% (P 0.47%)，the initial decomposition temperature(T5%) of PLA/MAH-DOPS is 315.4℃, which is lower than that of PLA(332.5℃）, the char residue is increased，the LOI value of PLA/MAH-DOPS is 28.6% , the vertical combustion grade is UL-94 V-0, both of THR and Av-EHC of PLA/MAH-DOPS were decreased.The?mechanical?properties results show that compared with pure PLA, the bending strength of PLA/MAH-DOPS increases by 2.66%, while the tensile strength and notch impact strength decreases slightly. MAH-DOPS exert a flame retardant effect in the gas phase and condensed phase by flame suppression and carbon formation respectively, and the gas phase flame retardant mechanism mainly.