Abstract: In order to study the effect of transient photocurrent response of the NPN bipolar transistor induced by neutron displacement damage, the experiments of neutron displacement damage effect and transient ionizing radiation effect were carried out on Xi’an pulsed reactor and “Qiangguang-Ⅰ” accelerator respectively, and the secondary photocurrent responses were obtained after different neutron fluences irradiation. The results show that as the irradiated neutron fluence varies, the amplitude and duration of secondary photocurrent have obvious discrepancy. It is considered that the response difference of secondary photocurrent amplitude induced by neutron displacement damage is related to the collector current of bipolar transistor. When transient dose rate is low, the current generated by transistor collector is small, the current gain decreases gradually with the increase of irradiated neutron fluence, and according to previous studies, the secondary photocurrent amplitude is positively correlated with the current gain of bipolar transistor, so the secondary photocurrent amplitude decreases gradually with the increase of irradiated neutron fluence. Whereas, when transient dose rate is high, the current generated by the transistor collector is large, the influence of irradiated neutron fluence on current gain can be neglected, therefore, the response difference of secondary photocurrent amplitude is not affected by the irradiated neutron fluence. The secondary photocurrent duration of transient ionizing radiation includes both generation and decay, the generation time of secondary photocurrent is basically same in the process of transient ionizing radiation after different neutron fluences irradiation, so the response difference of secondary photocurrent duration mainly depends on the decay time. The electron transport of secondary photocurrent passes through the whole space region from the emitter to the collector, therefore, all the minority carrier recombination processes will occur during the period. The decay time of secondary photocurrent is mainly affected by the minority carrier lifetime of the base region, and the ratio of the decay time of secondary photocurrent before and after neutron irradiation is directly related to the average lifetime of minority carrier in the base region. The ratio of the minority carrier average lifetime before and after neutron irradiation can be obtained by analyzing the proportion of the base current increment. By comparing the theoretical derivation with the experimental results, it is found that the decay time of secondary photocurrent decreases monotonously with the increase of irradiated neutron fluence. This work has important theoretical significance for improving the radiation hardening design of bipolar process devices under complex radiation environment.