For semi-crystalline material such as poly(lactic acid) (PLA), it is not easy to accelerate its crystallization, let alone to improve the flame retardancy and mechanical properties meanwhile. In this paper, a furan-phosphamide derivative (POCFA) was synthesized using furfurylamine (FA) and phosphorus oxychloride (POC). The POCFA was used as a flame retardant and nucleating agent in PLA. The flame retardancy and thermal stability of PLA/POCFA composites were investigated by the limited oxygen index (LOI), UL-94 vertical burning tests, cone calorimeter tests, and thermal gravity analysis (TGA). A small 2 wt% loading of POCFA increases the LOI of PLA from 19.4 vol% to 29.6 vol% and achieves the UL-94 VO rating. The POCFA decreases the amount of flammable gaseous compounds and increases the condensed components resulting in positive effect on transesterification, crosslinking and char formation of PLA chains. Moreover, the POCFA improves the crystallization rate and crystallinity of PLA indicating it is a good nucleation agent. In addition, the tensile strength of the PLA/POCFA composites is also improved.