More and more attention is paid to the spontaneous combustion accidents of oil which is induced by FeS. The thermal analysis experiments of FeS are finished with different heating rates of 2, 5, 8, 10, 15 ℃/min. The essence about the oxidation of ferrous sulfide in air is a process containing physical adsorption, chemical adsorption and chemical reaction of ferrous with the oxygen. Based on the experiments data, the kinetics of thermal decomposition of FeS is studied by model-fitting and model-free methods. It is shown that the computable results of the model-fitting are unstable and irresponsible. The kinetics parameters of FeS are calculated by the iso-conversional method. The apparent activated energy E is equal to(135.81±8.27)kJ/mol. Through the equation of Satava-Sestak, the kinetics of thermal degradation of FeS is researched. The reaction accorded with the Avrami-Erofeev (m=2) kinetic model is g(α)= ［-ln(1-α)］1/2. The apparent activated energy E is 148.43 kJ/mol and the apparent preexponential factor A is 3.82×109 K/s.