The high-voltage circuit breaker is commonly used electrical equipment, and the opening/closing coil is the key component of its operating mechanism. The coil current reflects the motion characteristic of the circuit breaker. This paper explains the structure of the opening/closing coil, and then describes a method to diagnose a circuit breaker fault based on detecting its opening/closing current. The method is applied to a 550 kV circuit breaker in a converter station. It shows that coil current detection of a circuit breaker is an effective method of identifying some hidden mechanical defects in the circuit breaker. Based on this method, some previously undiscoverable mechanical faults could be diagnosed, and the result is useful to the security and stability of the power system.