Catalytic decomposition of ozone was carried out in a 15-m high, 90-mm inner diameter downer reactor. Radial distributions of ozone concentration at different axial elevations under different operating conditions were measured. The results were explained from the flow structure experimental data and corresponding signal analysis in time domain and frequency domain. The aggregation status of solid particles, relative movement between the gas and solids, and the dynamic behaviour of clusters were considered to have a joint influence on the mass transfer and reaction process in the downer.