An automated damage diagnostic system for collecting plasma flash is developed to diagnose damage in a laser-induced damage threshold (LIDT) test system. Experiment is done to verify the accuracy of this system and analyze the relationship between the plasma signals and the damage morphologies. The results obtained by the system are found to be in excellent agreement with those obtained by the much laborious method of Normaski microscope. Results show that plasma signals above 1 V correspond to the damage morphology of surface discolorations with or without pits in their centers, and plasma signals below or just around 1 V correspond to the damage morphology of pits. The misdiagnosis is attributed to contaminations and air breakdown.