The thermal decomposition behavior of fire retardant viscose fiber and viscose fiber were studied by thermogravimetric analysis (TGA) under air atmosphere at heating rates of 10, 20, 30 and 40 o C/min. The activation energy and pre-exponential factor were calculated by using the Kissinger method, Flynn-Wall-Ozawa (FWO) method and Satava-Sestak method. The results show that the activation energy for the fire retardant viscose fiber calculated by Kissinger and FWO method was 102.51kJ/mol and 103.73kJ/mol, respectively. The activation energy for viscose fiber calculated by Kissinger and FWO method was 103.58 kJ/mol and 104.83kJ/mol, respectively. The kinetic mechanism function of fire retardant viscose fiber was G(α) = [(1+α) 1/3-1] 2 following a kinetic model of three-dimensional diffusion and the kinetic mechanism function of viscose fiber was G(α) = α 3/2 following the power function rule.