Abstract: Membrane bioreactor(MBR) technology has been widely used in water treatment industry because of its advantages like small footprint, good effluent quality, large biomass and low sludge yield. Since 2010, the number of MBR water plants has been increasing rapidly in China. Nowadays, the performance of MBR products in the early water plants has deteriorated seriously or reached the service life, cannot continue to meet the requirements of the treatment water quantity, and there is a need to add new membrane cassettes and replace membrane cassettes successively. This paper took a new assembly water plant project as an example, evaluated the operation of the original and new membrane cassettes after the addition of the membrane cassettes, and analysed the influence of the cassettes' difference on the system operation. The results showed that the new membrane cassettes ran stably for 160 days under the conditions of flux of 20.8 L/(m²·h), aeration intensity of 70 Nm³/(m²·h) and air-water ratio of 6, and the specific flux attenuation reached about 30%. At the same time, the scavenging energy consumption of the new assembly was lower than that of the original assembly. In addition, on the premise that the return flow cannot be improved, the high-throughput operation of the new packer may lead to uneven distribution of sludge concentration in the corridor and significant sludge concentration at the return end. Influenced by the pressure loss of the production pipe, the difference of the water production of each unit in the corridor resulted in different pollution degree of each unit.