Abstract: In order to further elucidate the influence of neutron pre-irradiation on the pulse gamma ray radiation effect of NPN bipolar transistors, the photocurrent response experiments of transient ionizing radiation were performed after NPN bipolar transistors were irradiated with different neutron fluences, and the variation trends of primary photocurrent amplitude and duration were obtained. It is found that the primary photocurrent response of transient ionizing radiation is mainly affected by the transient dose rate, minority carrier lifetime and carrier diffusion coefficient for the NPN bipolar transistors with good electrical performance and structure consistency. Combined the photocurrent response model and based on the transient dose rate range affecting the transistor parameters under irradiation with different neutron fluences, the relationship between primary photocurrent of transient ionizing radiation and the irradiated neutron fluence was analyzed from two stages respectively. It is concluded that the effect of neutron irradiation on carrier diffusion coefficient is different when the transient dose rate is within a certain range based on the photocurrent response model of transient ionizing radiation. The difference of influence factor D^1/2_\mathrma of the photocurrent response under irradiation with different neutron fluences is analyzed. It is found that the effect of irradiation with different neutron fluences on influence factor D^1/2_\mathrma has little difference. Therefore, the minority carrier lifetime is the most important factor affecting the primary photocurrent response of transient ionizing radiation. According to the expression of photocurrent response, the curve of primary photocurrent response with the minority carrier lifetime is obtained, and it is considered that the primary photocurrent response of transient ionizing radiation is basically unchanged with the degradation of minority carrier lifetime caused by neutron irradiation within a certain range, the analysis is consistent with the primary photocurrent response trend of transient ionizing radiation experiments after irradiation with different neutron fluences. In addition, based on the expression of photocurrent response of transient ionizing radiation, the magnitude of transient dose rate is also an important factor affecting the primary photocurrent response, so the relationship between primary photocurrent response and transient dose rate was also analyzed. The experimental results indicate that the change of neutron pre-irradiation fluences has no significant effect on primary photocurrent amplitude and duration of transient ionizing radiation, and the primary photocurrent response of bipolar transistor is positively correlated with transient dose rate, the experimental results are consistent with the theoretical analyses. This work can provide theoretical basis for the research of radiation hardening design of bipolar transistors in complex radiation environment of neutron and pulse gamma ray.