The statistical distribution of bipolar transistors' performance will change after ionizing radiation, from a symmetrical normal distribution before radiation to an asymmetric log-normal distribution. This statistical characteristic conversion lacks a clear physical image. In order to explain this transformation process from the microcosmic level, a large sample of ionizing radiation effect experiments for customized transistors are carried out to obtain the statistical characteristics of base currents and interface trapped charges before and after radiation, and the statistical characteristics of the two are found to be consistent. Based on the analytical physical model of the base current, it is found that the transformation of the base current statistical characteristics before and after radiation are originated from the transformation of the interface trapped charges. Based on the central limit theorem, the physical explanation for the statistical characteristics transformation of interface trapped charges before and after radiation is given, which comes from the physical model of interface defects in the form of multiple random variables.