The pyrolysis of polystyrene and the steam reforming reaction of its main product, styrene, are the core links in the steam reforming process of radioactive organic waste. To explore the mechanism of the steam reforming reaction and provide guidance for process optimization, this study constructed a heterogeneous gas-solid reaction kinetic model for the chemical reaction process of polystyrene resin, coupled it with the gas-solid flow characteristics in the fluidized bed, and conducted numerical simulation research on the chemical reactions. Thermodynamic analysis based on the Gibbs free energy minimization method clarified the effects of temperature, pressure, and steam-to-feed ratio on product distribution. A kinetic network incorporating key reaction pathways, such as styrene cracking, steam reforming, and water-gas shift reactions, was established using the shrinking core model and homogeneous reaction kinetics. Three-dimensional transient simulations of the reaction process were performed via Fluent software. The results indicate that the reactions are predominantly concentrated in the lower region of the fluidized bed, with product concentrations decreasing along the bed height. Hydrogen is the main gaseous product, and the relative error between the simulated values and experimental data is 10.01%, which meets the requirements of engineering accuracy. The entire reaction system exhibits an exothermic behavior, with heat release concentrated in the lower part of the bed. The findings of this study provide theoretical insights and simulation methodology support for the optimal design and operational control of steam reforming reactors for radioactive organic waste.