A novel method of capacitive current measurement（injecting one frequency signal measurement） is proposed for insulated neutral distribution networks. With a constant current signal injected into the open-delta side of the voltage transformer， the voltage of the open-delta side and the phase voltage of the secondary star side are measured. The distribution network capacitance and capacitive current to ground can be calculated in accordance with the injected current signal and the measured voltage signals. The proposed method avoids the influence of short-circuit impedance of the voltage transformer and the injecting signal frequencies. The results of dynamic power system simulation show that the proposed method is with high precision， and can be used in real-time without power supply interrupting.