To overcome the difficulty for Electronic Current Transformer（ECT） integrator to measure transient fault currents and to get synchronization signals for the sensor head precisely， the paper presents a new voltage-to-frequency conversion method. By using combination of a voltage-to-frequency converter（VFC） circuit， pulse counter and microprocessor， the method can obtain the integration waveform of input signals， i.e. the waveform of the current being measured， and avoid using conventional ECT integrator， sampling synchronization circuit and associated measurement errors. Simulation results show that the method can realize analog-to-digital conversion and integration simultaneously with high resolution and sampling rate in comparison with the conventional voltage-to-frequency method.