Abstract: This paper studied the situation of cultivating the biofilms with two different flow modes in an inner loop three-phase fluidized bed. The results showed that the diameter of the biofilm formed in a se quencing batch reactor (SBR) could increase to about 200~250μm, and that formed in a continuance reactor could arrive at 100μm. The former biofilm is smooth and compact, but the later has a higher activity. A lower initial loading rate makes for suspension membrane, and in the successive process, the loading rate can not be excessively low. Otherwise filamentous colonies expand because of the low loading rate. The biomass density of active sludge with SBR mode could increase to 9.65 g·L^-1, 92.7 % of which is attachment biofilm, and the loading rate of COD with continuance mode could arrive at 11.45 kg·m^-3·d^-1.