Abstract: The anti-pumping-circuit of circuit breaker is mainly composed of several relays and auxiliary nodes. The cooperation between relays is the premise to ensure the correct action of the anti-pumping-circuit and the opening and closing circuits. This paper introduces a bus trip accident in a substation, and makes a comprehensive analysis from all aspects, summarizes a hidden danger of the anti-pumping-circuit of circuit breaker, and confirms that the hidden danger is that the relay devices of the anti-pumping-circuit do not cooperate properly in time, resulting in the bus trip accident. Finally, the paper gives the solutions to the hidden danger.