The electrical and dielectric properties of a commercial ZnO-based varistor ceramics were measured in the process of 8/20 μs impulse current degradation up to 5000 times of impulses. The characteristics of U-I in the region of low current and the loss tangent were mainly investigated. It was found that the varistor voltage U1mA will first increase rapidly with impulse times increasing, and then remain stable and finally decrease sharply. Furthermore, a new loss peak emerges at -100℃ in the dielectric spectra after 600 times of impulse degradation, which suggests a new trapping behavior introduced in the degradation process. Activation energy of the new peak first decreases, then becomes independent on impulse times. It was also found that the non-linear coefficient was more sensitive to the degradation process than varistor voltage U1mA. These phenomena have not been reported before, which revealed that the variation of the neutral Fermi level between positive bias Schottky barrier region and reverse bias Schottky barrier region is the origin which leads to the degradation of electrical properties.