Based on the impedance sensor, we design a instrument for measuring water rate in crude oil. The least square method is employed first, Based on the measured data, the temperature coefficient of the impedance sensor is obtained , then the fitting curves of the relationship between moisture content and voltage at reference temperature is achieved. The impedance sensors now avaliable usually show the charge accumulation effect after working for a long time, which impacts the measuring precision of moisture content. In this paper, the discharge circuit is designed to discharge the stored charges regularly under the control of the microcontroller. Thus it provides an effective method to solve the problem of measurement error caused by charge accumulation. The experimental results show that this method can design the measurement of moisture content of crude oil with the high precision.