Abstract: Gas insulated switchgear（GIS） circuit breakers are widely used in extra-high or ultra-high voltage power grids due to such advantages as low cost, easy installation, and good seismic performance.Aiming at an accident of internal breakdown and discharge in a 750 kV GIS circuit breaker, from the aspects of on-site inspection, equipment disassembly inspection, electrical, thermal, and mechanical simulation analysis, the root cause of internal discharge in the circuit breaker was analyzed.The corresponding measures were proposed.