The circuit breaker is one of the most important equipment in power system, and the characteristics of opening and closing are the key indexes to evaluate the operation of circuit breaker. If the circuit breaker refuses to break down in the process of operation, it may cause a large area power failure, even threaten personal safety. Aiming at the opening breakdown of the circuit breaker in a substation during the operation mode conversion process, the causes of the opening breakdown of circuit breaker are comprehensively analyzed in this paper. The effective measures are taken to ensure the reliability of its opening, and corresponding rectification measures are put forward. A method of determining the opening failure probability of the circuit breaker is proposed. According to the opening failure probability, the hidden dangers that may lead to the opening failure of the circuit breaker are eliminated in advance, and the opening failure of the circuit breaker is effectively prevented.