In the present study, the thermal decomposition characteristics of polyethylene (PE) in oxygen-free and low oxygen content circumstances were examined by molecular dynamic (MD) simulations at atomic scale using reactive force field (ReaxFF). Temporal evolutions of species were captured reasonably during the processes of thermal decomposition. The effects of oxygen content, temperature and heating rate were also analysed. In addition, the kinetic properties were predicted with reliable parameters. The results show good agreements with the available ones, which illustrate that the species with two carbon atoms are the vast majority of final products. Higher oxygen content and temperature promote the generation of small molecules with carbon atom number less than or equal to 10. In the presence of oxygen, greater activation energy span and reaction order are calculated with lower adjust R-2, which indicates complex reactions according to kinetic analysis. The initial decomposition temperature of PE is proportional to the heating rate owing to heat transfer lag.