A method for determining delay time of undervoltage protection of a low-voltage circuit breaker for a distribution network line comprises the steps: (1) gathering substation switch protection fixed value information of all lines on a 10 kV bus line of an upper 10 kV line supplying power to the low-voltage circuit breaker, and selecting a plurality of calculated fixed value sets according to the principle of different time fixed values; (2) obtaining a system impedance value of the 10 kV bus line in a minimum operating mode; (3) according to the calculated fixed value sets and related information, using the method to calculate the corresponding minimum voltage of the low-voltage circuit breaker at the 10 kV bus line; and (4) determining the optimal delay time of the undervoltage protectionof the low-voltage circuit breaker by comparing data of a fault voltage fixed value sets and undervoltage protection operation voltage of the low-voltage circuit breaker. According to the method, theundervoltage protection delay time of the low-voltage circuit breaker is determined by considering the cooperation with the protection of the upper 10 kV line, and the scientificity and effectivenessof the setting of the low-voltage circuit breaker undervoltage protection delay time are improved.