Abstract: During the operation of the pneumatic mechanism circuit breaker in a 330 kV substation, continuous pressurization of its pneumatic operating mechanism was observed, with a sustained increasing trend. Based on field equipment inspections and disassembly analysis of the pneumatic circuit, the incident process was thoroughly investigated. This report details the procedures for defect investigation, analysis, and remediation. Rectification measures and recommendations addressing this safety hazard are proposed in accordance with the operational principles of the circuit breaker's pneumatic mechanism.