In this work, a synergistic effect strategy was explored for fabricating hydrophobicity, phosphorus- and nitrogen-containing layered double hydroxides nanosheets (LDHs) based flame retardant (LDHs-K@DM). This fabricating process was achieved by amination of the surface of LDHs and, subsequently, interfacial self-assembly with high P content of diethylenetriaminpenta (methylene-phosphonic acid) (DTPMP) and high N content of melamine (MEL). The chemical construction and thermal stabilization properties of LDHs-K@DM were confirmed via different characterization approaches. The results revealed that LDHs-K@DM was successfully prepared and the surface of LDHs-K@DM was transformed into hydrophobicity with high water contact angle. Moreover, LDHs-K@DM had better thermal stability. After that, LDHs-K@DM was molten compounded with biobased polylactic acid (PLA). It was suggested that LDHs-K@DM were uniformly dispersed in the PLA matrix and could accelerate the decomposition of PLA to form char residues. Most importantly, the addition of 20 wt% LDHs-K@DM into PLA can endow the composite to achieve a desirable UL-94 V-0 rating and the limiting oxygen index value of 30.9%. Finally, the synergistic effect flame retardant mechanism of LDHs-K@DM was comprehensively analyzed. This synergistic decorating strategy for preparing LDHs-K@DM flame retardant is promising for industrial production.