A novel reacting P-N intumescence flame retardant, trioxo (3-triethylphosphite- 5-chlorine- 1-triazine) glycerin (TTCTG), has been synthesized and the molecular structure was characterized by FTIR, H NMR, and 31P NMR. The thermal degradation property of TTCTG was assessed by TGA, which revealed that it was good thermal stability on the high temperature condition. The flame retardancy of cotton fabrics with different TTCTG content was investigated by using TGA, vertical flammability test, LOI, and SEM,the results showed that when the content of TTCTG was 20%, the limiting oxygen index of 20%TTCTG-CF was up to 27.3%, the flame retardancy reached the class B1 of national standard of flame retardant fabrics．The TGA results indicated TTCTG could promote the rapid decomposition of cotton fabric into char at 284 ℃, SEM showed that the surface of 20%TTCTG-CF formed the dense expansive carbon layer after combustion. After washing for 20 times, the LOI of 20%TTCTG-CF was still as high as 26.4%, reaching class B1 of national standard of flame retardant fabrics. The structure of 20%TTCTG-CF after 20 washing times was analyzed by SEM and FTIR, and the results showed that TTCTG and cotton fiber were linked by covalent bond, which gave cotton fiber excellent and lasting flame retardancy.