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In order to accurately diagnose the degradation degree of ZnO varistor, a new method，return voltage measurement， is presented. Samples of ZnO varistors were impacted by impulse currents, and various diagnostic techniques are employed to investigate the degradation degree of those samples. It is proved that the conclusions drawn from return voltage measurement are consistent with the traditional varistor voltage〖WTBX〗 〖WTBZ〗(〖WTBX〗U〖WTBZ〗1mA) and residual voltage method. Besides, two main parameters getting from the return voltage measurement appear to decrease sharply after the impacting process, which is also consistent with SEM experimental results: the average grain size decreases and the conductivity increases. The experimental results further prove the correctness of return voltage measurement, which has a certain reference value for improving the accuracy of ZnO varistor diagnosis.