In this work, a novel single macromolecular intumescent flame retardant (DT-M) was prepared by self-assembly of diethylenetriamine penta-(methylenephosphonic) acid with melamine, and modified layered double hydroxides (LDH) were also prepared by intercalating phytic acid into LDHs. The chemical structures of DT-M and phytic acid intercalated LDHs (PA–LDH) were fully characterized by different analytical instruments, and then were introduced into PLA by melt compounding to prepare a flame retardant biodegradable PLA composite. The fire performance evaluation of PLA composites by limiting oxygen index (LOI), vertical burning (UL-94) and cone calorimeter tests indicated that the introduction of 14% DT-M and 1% PA–LDH increased the LOI value from 19.4 to 38.9%, upgraded the UL-94 rating from no rating to V-0, and decreased the peak heat release rate from 812 to 301 kW m−2. The analysis for decomposition products of PLA composite and the observation of morphology of the char suggested that DT-M and PA–LDH took effects in both condensed phase through promoting char formation and gas phase through releasing inert gases.