The formaldehyde-free phosphorus-nitrogen flame retardant was synthesized by using dimethyl phosphite, acrylamide and cyanuric chloride and the cotton fabric was flame-retarded by the rolling and baking process. The structure of the intermediate and flame retardant was characterized by infrared spectroscopy. The structure, surface morphology and elemental composition of the finished fabric were characterized by infrared spectroscopy and electron microscopy. The flame retardant properties of the finished fabric were tested by limiting oxygen index, vertical burning and cone calorimetry. The thermal stability of the finished fabric was tested by thermogravimetry. The results show that the flame retardant is successfully finished on the fabric, and the maximum thermal decomposition rate of the finished fabric is reduced, and the carbon residue rate is improved. The limiting oxygen index of the finished fabric increased from 18% to 31%, the afterburning time, the smoldering time decreased from 9 s, 25s to 0 s, and the damage length decreased from 300 mm to 74 mm. The maximum heat release rate (PHRR) of the finished fabric was reduced from 203.5 kW?m-2 to 57.9 kW?m-2, and the total heat of release (THR) was reduced from 6.0 MJ?m-2 to 2.7 MJ?m-2.