The flow rate of gas-water mixture in the membrane reactor could be measured with laser Doppler velocimetry（LDV）. The impact of deflector，aeration intensity and flow rate of effluent on the shear forces of the membrane surface was investigated. The results showed that the deflector significantly increased the tangential velocity and turbulence of the mixture on the membrane surface. As a result，the shear force on the membrane surface was enhanced and in turn the formation of cake layer was delayed，thus alleviating concentration polarization. The vertical and horizontal velocities increased with increasing effluent flow rate. Also，aeration intensity affected the flow rate of the mixture remarkably. It was found that mean and pulse velocities were positively proportional to aeration intensity in logarithmic relationship，respectively，indicating significant effects of aeration intensity on turbulence and shear forces.