A lightning overvoltage monitoring system for a substation secondary system is characterized in that a low-damping series resistance-capacitance divider acquires a surge overvoltage signal, a parallel resistance-capacitance divider divides voltage of the surge overvoltage signal, voltage amplitude of surge overvoltage is lowered, part of an overvoltage division signal enters a trigger circuit for overvoltage judging, whether overvoltage signal acquisition is required or not is determined, the other part of the overvoltage division signal enters a signal processing circuit for impedance isolation and low-pass filtering, when triggered, a data acquisition card acquires an overvoltage signal output by the signal processing circuit and transmits the overvoltage signal to a background computer for overvoltage waveform statistics and analysis, action reliability of an arrestor is judged according to analysis results, maintenance for the arrestor of the substation secondary system is thus guided, and running safety and normality of the substation secondary system is ensured.