The ring opening reaction of polycyclic aromatic hydrocarbons (PAHs) is one of the rate-determining steps of coal gasification within supercritical water, while the involvement of water remains debated between addition and hydrogen abstraction theories. In this study, reactive molecular dynamics (MD) simulations were performed to explore the ring opening reaction of naphthalene, the simplest PAH, at temperatures of 2500—2700 K. Species and elementary reactions were quantitatively extracted from bond order trajectories using an in-house program. Reaction path analysis shows that ring opening paths of naphthalene mainly include thermolysis, H atom abstraction, and OH radical addition. Flux analysis indicates OH radical largely from the H + H₂O = OH + H₂ reaction, while the water dissociation H₂O = OH +H reaction is near equilibrium, seldom contributing to the OH source. This atomistic kinetics analysis is intended to help the modelling of supercritical water gasification of coal.