Cotton fabrics require flame retardant finish as they are used extensively and easy to be ignited causing fire. In this paper, cotton was treated with Poly(oxy-1,2-ethanediyl)methylphosphonate(PEMP) as a flame retardant, its properties, such as breaking elongation, flame retardancy, thermal decomposition, fire behavior and water durable, were compared with pure cotton. It was found that loading 20% PEMP, the cotton’s breaking elongation went down from 23.6 % to 18.2 %, LOI rose from 20.4 % up to 30.6 %, after flame time and smoldering time both dropped to 0 s from 54.8 s and 61.2 s respectively, damaged length fell to 11.5 cm from 30 cm, the maximum thermal decomposition rate dropped to 12 %/min from 18 %/min, the residue under 700 ℃ enhanced from 9 % to 24 %, total heat release fell to 0.6 MJ/m2 from 4.2 MJ/m2, the LOI was still 27.9 %. All the results showed that PEMP is a good flame retardant and a charring agent for cotton and can reduce evidently heat release of cotton in fire that can suppress fire behavior and race against time for fire fighting and rescue under fire.