In order to overcome the performance barriers of polylactic acid (PLA), a multifunctional layered chitosan derivative, DAMC-Al, is synthesised and used as an intumescent flame retardant. The acid sources used in this work are 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and amino trimethylene phosphonic acid, which act synergistically to facilitate the conversion of chitosan to carbon. The combustion tests demonstrate that the PLA/ 5DAMC-Al exhibits excellent flame-retardancy with a limiting oxygen index of 30.3 % and a UL-94 flammability rating of V-0. The heat release rate and total heat release are significantly reduced compared to pure PLA, and the residual carbon expands rapidly. In addition, the composite has improved toughness while maintaining the stiffness of the PLA substrate, with a 31.0 % increase in elongation at break. DAMC-Al can also effectively block the transmission of 95 % of ultraviolet lights, protecting the PLA macromolecular chain from high-energy ultraviolet damage. Actually, the ultraviolet protection factor of PLA/5DAMC-Al is determined to be as high as 51.43. Furthermore, the soil degradation tests show that PLA/5DAMC-Al has the same excellent natural degradation properties as PLA.