A spiro-cyclic phosphate SPMA was synthesized from N-Methylol acrylamide (NMA) and pentaerythritol spiro phosphoryl dichloride(SPDPC).The reaction time and reaction mechanism were determined by tracking the reaction process using ultraviolet spectrum. It was found that the reaction could achieve the optimum state of chemical equilibrium in 1h. At first, the –NH- of NMA quickly reacted with SPDPC, then the –OH slowly reacted to form the final product by equilibrium reaction. The melt point of the product SPMA is 126～130 ℃. The thermal stability of SPMA was evaluated by Thermogravimetry and it was found that SPMA has better thermal stability and carbonization performance with 313℃of the initial decomposition temperature, 343℃ of the maximum weight loss temperature (Tmax) and 32.6% of residue under 800℃. With SPMA flame retardant finishing of cotton cloth and the addition was 5%, the limiting oxygen index increased by 2%, the combustion and smoldering time reduced by more than 70%.