DC circuit breakers (DCCBs) are critical components in modern DC power distribution systems, where ZnO-based varistors are widely employed as energy absorption and dissipation devices during fault current interruption. Despite their extensive use, the degradation behavior of varistors under repetitive unipolar electrical impulse stress—typical of DCCB operation—remains insufficiently understood. This study systematically evaluates potential DCCB varistor materials based on their nonlinearity and current density performance, and introduces a fully automated accelerated degradation test platform. Using this platform, 975 varistors from eight models were tested over more than two million DCCB opening events, generating a comprehensive database of standardized degradation patterns. Impedance measurements before and after testing reveal degradation-related variations in equivalent capacitance and identify a short-term recovery effect during cooling intervals. The observed behaviors are interpreted through proposed macroscopic and microscopic degradation mechanisms, offering new insights into varistor degradation and its mechanism in DCCB applications.