n-Heptane is the most important straight chain paraffin in the fossil-fuel industry. In this work, pyrolysis of n-heptane at high temperature is investigated by a series of ReaxFF based reactive molecular dynamic simulations. The pyrolysis correlated intermediate reactions, important product/intermediate distributions, and corresponding kinetics behaviors are systematically analyzed at atomistic level. The results indicate that the entire pyrolysis process is radical-dominated. The unimolecular dissociation is the main pathway of n-heptane decomposition. Initiation of the decomposition is mainly through C-C bond fission. Central C-C bonds would dissociate prior to the terminal ones. Besides, the Rice-Kossiakoff theory is proved for the pyrolysis of n-heptane at the atomistic level. To give a better description of the pyrolysis behavior, some alkane related intermolecular reactions should be considered in the mechanism. The apparent activation energy extracted from the present simulations is 43.02-54.49 kcal/mol in the temperature range 2400-3000 K, which is reasonably consistent with the experimental results.