A novel phosphorus/nitrogen intumescent flame retardant, namely 6-chloro-4-(diethylamino phosphorate phosphoryl chloride)-2-(sodium 4-aminobenzensulfonate)-1,3,5-triazine (APPCABT), has been synthesized and characterized by elemental analysis, FT-IR, H-1-NMR and P-31{H}-NMR spectroscopies. To investigate the flame-retarding properties of the treated cotton fabrics, APPCABT was applied to the chemical treatment of the cotton fabrics surface. The flame-retarding performance of the treated samples with APPCABT was evaluated by limiting oxygen index (LOI). The thermal degradation behavior of the treated samples was studied using thermogravimetric analysis (TGA). The surface morphology of the treated samples was observed by scanning electron microscopy (SEM). These results show that APPCABT is both an intumescent flame retardant with high efficiency and an excellent char-forming agent for the flame-retarding treatment of cotton fabrics.