The electrostatic method with ring electrodes embedded in pipe walls is widely used for the measurement of dilute gas-solid two phase flow. The signal from electrostatic sensors contains not only the information about the induced charge when the charged particles move through the electrode, but also the information about transferred charge when the charged particles impact on the electrode wall. A current source equivalent model is built to study the sensing mechanism. The mean value of the signal is mainly generated by charged particles impacting on the electrode and charge transferring.