Abstract: A novel phosphorus-nitrogen quaternary ammonium salts (PAHAC) was synthesized with phoshorus oxychloride and neopentyl glycol as main raw materials. PAHAC was characterized by FTIR, 1H-NMR and HRMS. Montmorillonite (MMT) modified with PAHAC (PAHACMMT) was prepared by ion-exchanging of Na⁺-montmorillonite (Na-MMT) and PAHAC. FTIR and XRD indicate that PAHAC is intercalated with Na-MMT and the layer spacing and the decomposition temperature of MMT modified with 20wt% PAHAC (20%PAHACMMT) is 1.87nm and 310℃, respectively. The results of TEM show that the intercalated-exploitated nanocomposite of 20%PAHACMMT/LDPE is formed by melting intercalation blending LDPE with 20%PAHACMMT. The  flame retardancy of phosphorus-nitrogenmontmorillonite/LDPE nanocomposites was investigated by cone calorimeter test. The results show that the flame retardancy properties of phosphorus-nitrogen-montmorillonite/LDPE nanocomposites are good. In comparison with LDPE, the peak heat release rates (PHRR) and the total heat release (THR) of 20%PAHACMMT(7%)/LDPE reduce by 21% and 9.5%, respectively. The results of SEM show that the compact and dense char is formed for 20%PAHACMMT/LDPE nanocomposite after combustion, which can inhibit burning effectively.