Through the pad method pure cotton was finished with a novel intumescent flame retardant imidazole spirocyclic phosphoramidate synthesized by ourselves. The influence of the proportion of flame retardant and other assistant agents and the curing temperature on finishing cotton limit oxygen index (LOI) was investigated through orthogonal experiment. The vertical burning and tensile performance were tested for eight flame-retardant cotton samples which LOI over 28% and the better flame retardant finishing process was determined. When the finishing liquor consist of 30% imidazole spirocyclic phosphoramidate and 1.5% catalyst and 2%cross-linking agent and the baking temperature was 170℃, the finishing cotton has 33.96% LOI and B1 flame retardant level with 0 s afterflame time and 0 s afterglow time and 64 mm damaged length and 312.50 N breaking force. The thermal gravimetric analysis showed that the finishing cotton’s initial thermal decomposition temperature brought forward 100 ℃, the maximum thermal decomposition rate was reduced, the residue at 600 ℃ increased to 30% from almost 0%. All above results explain that the cotton finished with imidazole spirocyclic phosphoramidate has good flame retardant effect.