Ion exchange resins are widely used in nuclear power plant water treatment systems to remove radioactive contaminants, control system chemistry, and reduce corrosion. However, there is no effective treatment method for industrial waste resin treatment. Supercritical water oxidation (SCWO) is a promising method to solve this problem. A novel packed reactor with counterflow heating for supercritical water oxidation is designed to treat ion exchange resins. A 3D simulation of the fluid dynamics and heat transfer in an oxidation process is performed to define the appropriate dimensions of the packed reactor. The turbulent flow is taken into account in the SST k-ω model. For chemical reactions, a three-step mechanism is created to describe homogeneous reactions. A Finite-Rate/Eddy-Dissipation approach is used to solve the multi-step reaction. The heterogeneous reaction (the solubilization of particles) is simplified as a series of mass sources. The porous media model is used to model the viscous and inertial resistance in the zone where the particles are packed. The mass fraction profile of each species (C7H6O3, C6H6O, C2H4O2) inside the packed reactor is obtained from the simulation. Outlet temperature is predicted with a deviation lower than 15%. The effects of the auxiliary heat source and the geometry of the reactor are focused on in the model. The research results provide important theoretical guidance for radiation waste treatment.