A mathematical mechanism of the n-pentane pyrolysis process based on free radical reaction model was presented. The kinetic parameters of n-pentane pyrolysis are obtained by quantum chemistry and the reaction network is established. The solution of the stiff ordinary differential equations in the n-pentane pyrolysis model is completed by semi implicit Eular algorithm. Then the pyrolysis mechanism based on free radical reaction model is built, and the computational efficiency increases 10 times by algorithm optimization. The validity of this model and its solution method is confirmed by the experimental results of n-pentane pyrolysis. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.