In a method and circuit for detecting high-voltage discharge of a gas-actuated fastener-driving gun, an attenuating unit attenuates a discharge voltage from the gas-actuated fastener-driving gun to generate a positive pulse signal. A signal processing unit processes the positive pulse signal to generate a processed pulse signal with an amplified amplitude. A determining unit outputs a control signal upon detecting that the processed pulse signal is greater than a predetermined voltage. A warning unit generates, based on the control signal from the determining unit, a warning output for indicating that the high-voltage discharge is abnormal.