Reactive force field (Reaxff) is a powerful method, which employs bond order/bond length formulism to describe bond breaking and bond reformation. In this work, a modification to the bond order formula was made and Reaxff parameters were re-optimized. The underlying idea of these modifications is to improve the energy gradient. Better agreements of the bond dissociation potential curves with the quantum mechanical curves were obtained on the basis of the aforementioned changes. Reaxff simulation was carried out to gain the understandings of the iso-octane pyrolysis. The apparent rate constants obtained using the newly optimized parameters fit well with the experimental results. The simulation results are in agreement with the existing experimental results. A maximum of C-2 hydrocarbons were found to have the largest percentage. The distribution of the iso-octane decomposition pathway was illustrated.