In addition to considering the cable load, voltage loss and thermal stability, the cable selection of the power distribution line should also be used for the sensitivity check of the circuit breaker of longdistance power distribution line. Therefore, the sensitivity of circuit breaker of longdistance power distribution line in tunnel is discussed based on the engineering practice. And then the necessity and importance of sensitivity check of circuit breaker in the safety check of longdistance power distribution line is introduced; and the technologies, i.e. enlarging the cable crosssection, adding the circuit breaker with residual current protection capacity and reducing the setting value of the action current of the circuit breaker based on keeping power distribution and protection function, should be adopted to improve the sensitivity of the circuit breaker when the sensitivity checking result does not meet the requirements.