Biodegradable polylactic acid (PLA) is a typical green polymer material. The high flammability, however, with a low limit oxygen index (LOI) of about 20%, limits its application safety. A graphene-supported phosphorus-nitrogen combined flame retardant (named CN-rGO) was synthesized using 2-carboxyethyl phenyl phosphinic acid, carbamide, and reduced graphene oxide (rGO) as raw materials. The PLA/CN-rGO composites were prepared via melt blending, and the flame retardant properties and mechanism of action were studied and analyzed. The test results based on limiting oxygen index, vertical combustion level, and cone calorimetry indicate that CN-rGO shows a high flame retardant efficiency. The LOI of PLA/CN-rGO, with the addition of only 1% mass fraction, is significantly increased to 37%, reaching difficultly-flammable level. The vertical combustion level achieves V-0, showing self-extinguish phenomenon. Furthermore, the peak heat release rate (PHRR) and total heat release (THR) are decreased by 40.2% and 23.1%, respectively. Mechanism studies reveal that CN-rGO exhibits flame retardancy in condensed and gaseous phase, comprehensively improving flame retardant performance. In addition, the surface specific resistance of PLA/CN-rGO reaches the anti-static level, indicating the multi-functionality of CN-rGO.