A novel phosphorus monomer (PDHA) was synthesized and incorporated onto the surface of cotton fabric via UV-induced graft polymerization to improve flame retardancy and thermal stability. The structure of PDHA was characterized by H-1 nuclear magnetic resonance (H-1 NMR), P-31 NMR, and Fourier transform infrared spectra (FTIR) and the grafting processes were followed by IR (ATR) spectroscopy and scanning electron microscopy (SEM). The thermal stability and flame retardancy of the treated fabrics were studied by thermogravimetric analysis (TGA), thermogravimetric analysis/infrared spectrometry (TGA-IR), and microscale combustion colorimeter (MCC). Test performed on real time FTIR analysis for treated cotton showed that the coatings containing phosphorus could catalyze the dehydration and char formation of cotton textiles at a lower temperature. The enhanced flame retardant action may be caused by the thermal decomposition of phosphate acrylate producing acidic intermediates, which could react with fabrics to alter its thermal decomposition.