The decomposition of H2O2 by a novel supported γ-FeOOH catalyst was performed in a continuous fluidized-bed reactor. This catalyst has been successfully used in the treatment of organic contaminants with H2O2 in our previous work. In this study, we attempted to determine the effects of pH, H2O2 concentration, and catalyst concentration on the decomposition of H2O2. An approach, we regarded this reactor as a continuous-flow stirred-tank reactor, was applied to investigate the kinetic behavior. At low H2O2 concentration, the decomposition rate of H2O2 was found to be proportional to both H2O2 and catalyst concentrations. At high H2O2 concentration, however, the rate decreased with the increasing H2O2 concentration. This can be explained by the substrate inhibition model. The large difference in the observed first-order rate constants under various pH values was also modeled. The model agreed well with the experimental results.