Abstract: To address the latent incomplete closing fault of the spring operating mechanism circuit breaker, this paper conducts an analysis through on-site and factory inspection tests, as well as multi-condition comparative tests. The results indicate that the root cause lies in the undersized flow passage cross-section on the closing side of the buffer in the associated spring operating mechanism. During the closing process, excessive oil damping causes a significant portion of the closing operation energy to be ineffectively dissipated by the buffer. This directly leads to insufficient residual closing operation energy and a marked degradation in the mechanism’s anti-disturbance capability, ultimately resulting in incomplete closing. The buffer of this circuit breaker model exhibits a family defect and requires optimization and upgrading. It is recommended that additional technical parameter assessments, such as closing overshoot and terminal velocity of the buffer, be incorporated into factory acceptance tests and diagnostic tests for spring operating mechanism circuit breakers to enhance operational reliability.The research findings provide a valuable reference for equipment manufacturing, operation and maintenance, overhaul, testing, and acceptance.