The membrane filtration will be the core technology in the future. However the technical bottleneck of membrane filtration is the “fouling”, that results in effluent flux decreasing and the Trans-Membrane Pressure (TMP) increasing. Finally lead to the membrane plugging and lower production rate, even then the membrane damage and diminish the operation time. Therefore, we will develop the device of ultrasonic time domain reflectometer (UTDR), in order to analysis membrane fouling potential and monitor the cake thickness, concentration polarization phenomenon. From the experiment result, the signals from UTDR are related to fouling forming and cake thickness. By this way, we can find the different fouling substances have different curve parameter, so we also can utilize related parameter to calculate the cake thickness under different experiment operation time and different filtration resistance situation to determine the optional membrane backwash time.