This work attempts to investigate initial reactivity for pyrolysis of a rocket propellant fuel (RP-1) by reactive molecular dynamics simulations with ReaxFF force field. The initial reactivity differences between a 3-component surrogate model and a more complex 24-component model have been observed in a series of heat-up and isothermal pyrolysis simulations performed using the GPU-enabled code GMD-Reax. The RP-1 conversion in the 3-component surrogate is slower than that of the 24-component model. The maximal weight fraction difference for RP-1 consumption can be up to 21.2% in heat-up simulations and 22.3% in isothermal simulations. The reaction analysis facilitated by the code VARxMD further reveals the differences of pyrolysis intermediates, products, and reaction pathways between the two RP-1 models. Normal paraffin reactions are similar between the two RP-1 models owing to the similar fuel structures of normal alkanes. For branched paraffin reactions, the pyrolysis of the multi-branched fuel component of iso-cetane in the 3-component surrogate will produce 2-methylpropene, which is not a major pyrolysis product in the 24-component model mainly due to lack of quaternary carbon with methyl side chains in the branched paraffin components. Compared to the reactions of methylcyclohexane, the only cycloparaffin in the 3-component surrogate, more versatile ring opening reactions of cycloparaffins can occur in the 24-component model that will generate more dienes and cyclohexene from the double-ring fuel structures. This work suggests that the reactive molecular dynamics simulations of multicomponent model with rich chemical structures closer to real fuel components have the potential as an alternative approach for evaluating reactivity in fuel pyrolysis.