The flame-retardant finishing of cotton with pentaerythritol phosphate and small molecule organic amine, such as diethylenetriamine, triethylenetetramine, diethanolamine, or isopropanolamine, was studied. The mechanical and flame-retardant properties and thermal stability of cotton fabric before and after flame-retardant finishing were investigated. Results revealed that the longitudinal and latitudinal breaking strength increased. The limiting oxygen index increased from 18.7 % to 37.9–39.1 %. Vertical flame burning time and afterglow time decreased from 35.1 and 8.4 s, respectively, to 0 s. The damage length decreased from 30 cm to 7.1–8.4 cm. At 700 °C, the residual carbon increased from 17.69 % to 41.48–41.56 %. The experimental results show that the flame-retardant system has good flame retardant and carbonization effects on cotton and good foaming property for combustion. The experimental operation is simple, rapid, and has broad market prospects.