By means of experiments, the aging situation of ZnO varistor under the effect of impulse-current was observed, also, parameters (i.e. the change rate of U1mA and power loss) which reflect aging degree was measured. The relationship between the times of impulse current -which is under varies current peaks, waveforms, varistor temperatures- and ZnO varistor's life-time is also analyzed. Therefore, a discipline is concluded, which reflects the relationship between the times of large current going through arrester and its life-time. And the times of current with a certain value which is larger than arrester's nominal discharge current, can be regarded as a criterion as the ending of arrester's life-time. In the end, according to this conclusion, an on-line life-time monitoring device of surge arresters with series gap was designed, made and tested.