In this study, two phosphorus-based flame retardants diethylenetriamine trimethyl diphosphonate lysine (APTA) and a tetrakis(hydroxymethyl)phosphonium sulfate prepolymer with urea (DUPT) were synthesized. The structures of these compounds were characterized via nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR). FTIR and scanning electron microscopy (SEM) analyses revealed that DUPT crosslinked APTA onto cellulose, which was pre-processed with diethylenetriamine dipropylene oxide (NAED) to introduce NH groups through PCN bonds. The APTA/DUPT-30 cotton exhibited a limiting oxygen index (LOI) of 36.4 % and a damaged length of 4.8 cm. After 50 washing cycles, the treated cotton maintained an LOI of 30.5 % and a damaged length of 5.4 cm. These results indicate the exceptional flame retardancy and washing resistance of the treated cotton fabrics. Moreover, the thermogravimetry (TG) curves indicated that APTA/DUPT altered the degradation process of the cotton fabrics. The TG-FTIR data and residue analysis from the vertical flame-retardant test confirmed that the treated cotton utilized a condensed-phase flame-retardant mechanism. Overall, this approach effectively imparts excellent durable flame retardancy to cotton fabrics through stable PCN bonds, eliminating the need for specialized equipment.