The internal-loop granular sludge bed nitrifying reactor is a new type of aerobic nitrifying equipment and has taken on a good potential for nitrification. The critical aerating flux for liquid loop and critical aerating flux for fluidization of granular sludge are two important parameters for its operation. The relationship between liquid superficial velocity in riser (U1r) and aerating flux(Ugr) was studied, the model parameters were measured by experiment, and the relational expression was established. According to the model, the critical aerating flux for liquid loop and the critical aerating flux for fluidization of granular sludge were calculated as 1.017cm/min and 2.662cm/min, respectively. The experimental data from reactor operation showed that the two calculated critical aerating fluxes near the practical values. So they could be used to direct the design and operating optimization for the internal-loop granular sludge bed nitrifying reactor.