Abstract: Fault diagnosis and analysis were carried out for typical cases of abnormal opening of high-voltage circuit breakers. Through SF₆ gas pressure monitoring of the circuit breaker body, insulation performance test of the circuit breaker, mechanical performance test of the circuit breaker and factory disassembly, the cause of the accident was found to be due to abnormal spring function of the circuit breaker operating mechanism. The analysis method of breaker opening and closing accident and the preventive measures in operation and maintenance are proposed.