Polylactic acid (PLA) is recognized as a promising alternative to traditional petroleum-based plastics due to its excellent biodegradability and well-balanced mechanical properties. Nevertheless, the disadvantages of PLA such as flammability in fire, susceptibility to UV light attack, and slow natural degradation rate limit its application and recovery in high-security areas. In this work, a spherical chitosan-based additive DMPC-Al with mirrorsymmetric internal structure was assembled by layer-by-layer electrostatic reactions, resulting in PLA characterized excellent comprehensive performances. When 7 wt% DMPC-Al was added into PLA, the LOI value of the composite PLA/7DMPC-Al was increased to 29.6%, and UL-94 reached V-0 grade without any molten droplets. The peak heat release rate and total heat release rate were reduced by 13.5% and 16.2%, respectively, and the carbon layer was highly self-expanding. In addition, the UPF of PLA/7DMPC-Al was increased to 34.45 from 0.45 of pure PLA, blocking most of the UV light attacks and extending the service life of PLA. Surprisingly, DMPC-Al actually improved the impact toughness of PLA by 38.5% and facilitated PLA to work continuously when drawing large curved shapes by 3D printing. More importantly, the introduction of DMPC-Al changed the sensitivity of PLA to water and provided sufficient energy for microbial growth, thus accelerating the degradation rate of PLA in the soil under abandoned buildings. This work provides a practical and feasible strategy to achieve multifunctionality of degradable plastics.