Natural pyrite catalysts were utilized in fluidized bed reactor for dye degradation in presence of hydrogen peroxide, which is famous to heterogeneous Fenton reaction. This process plays an important role in wastewater treatment processes and it is more effective when occurs in this kind of reactors. A novel kinetic model for Acid yellow 36 (AY36) degradation by heterogeneous Fenton process, in a fluidized bed reactor has been developed. By evaluating dissolved oxygen (DO) concentration in effluent during the process, a new parameter named effective reaction time is introduced, which could describe the relation of DO concentration and dye degradation, so the prediction of DO concentration by the model is of great importance toward the understanding of process performance. Neglecting mass transfer phenomenon from kinetic models eventuated in incorrect estimation, consequently, in this model, both reaction and mass transfer mechanism have been considered, which forecast the changes in effective factors like pH, DO concentration and dye removal efficiency simultaneously. The model results adequately coincide with the experimental results, which declare the validity of the modified kinetic model. Display omitted •A novel kinetic model for dye degradation by heterogeneous Fenton in a fluidized bed reactor.•Mass transfer is considered in the model.•A new parameter named effective reaction time is introduced.•Effective reaction time could describe the relation of dissolved oxygen concentration and dye degradation.