Abstract: Under operating conditions, the contact which determines whether there is a heating phenomenon inside the arc-extinguishing chamber of the high-voltage porcelain post circuit breaker carries the corresponding circuit current. In this paper, contact erosion fault of a 126 kV circuit breaker used in a 110 kV substation was analyzed through the action process wave-form analysis, on-site disassembly inspection, theoretical calculation and multi-physical field simulation, which clarifies the failure mechanism and the cause of erosion. In addition, through the same batch of circuit breakers to carry out hidden defect investigation, it is clear that the main reason for the hidden defect was the lack of testing of the circuit breaker after the replacement of the structure. Finally, this paper combines the fault mechanism and simulation analysis results to give engineering recommendations which can provide support for the avoidance of faults caused by poor contact of the internal contacts in circuit breakers.