Self-forming dynamic membrane coupled bioreactor is a technology using coarse pore-sized material modules to separate solid and liquid in the activated sludge. The cake layer formed on the coarse pore-sized material, i.e. dynamic membrane, can remove turbidity efficiently and some organic pollutants. Its biological activity can be studied by microelectrode. For measurement in-situ, a small bioreactor is designed to simulate the real filtration process. A microelectrode with micromanipulator is used to measure the dissolved oxygen profile in the dynamic membrane. The results indicate that bubble emerged in the dynamic membrane during operation of bioreactor. The dissolved oxygen concentration decreased with the increasing depth, and fell to 0 at 2.0 - 2.5mm depth. The specific oxygen uptake rates of the surface part of dynamic membrane were calculated to be 34.3, 10.6 and 12.4 mg/(g x h) respectively at the 1st, 5th and 8th day of the filtration.