The flow characteristics and effects of aeration volume on velocity distribution of the fluid in primary reaction zone of multi-flora biological wastewater treatment reactor were studied with clean water. Experimental results demonstrated that the fluid in primary reaction zone formed non-symmetrical cycle flow in the vertical plane with lower flow velocity in the central area and higher flow velocity in the peripheral area under the lifting action of the bubbles from perforated aerated pipe. As the aeration volume varied from1.0 m3/h to 2.2 m3/h, the flow velocity of central points had lower increased rate contrary to the peripheral points with higher increased rate in primary reaction area. Thus, the decreasing gradient of flow velocity of the primary reaction zone from the outer race to the center could be changed by controlling the aeration volume. For the design and operation of multi-flora biological wastewater treatment reactor, the results had important guiding significances that were listed out.