Abstract: The DC gain of bipolar transistor will degenerate after neutron irradiation. In the range of flux of 10⁹-10¹⁶ cm^-2, the reciprocal change of DC gain is linearly related to the neutron flux. The high temperature annealing of the bipolar transistor with DC gain degradation can restore the performance of the bipolar transistor which has been damaged by radiation. In view of this, the bipolar transistor was used in reverse engineering, and the neutron flux detector was fabricated. After calibration, the neutron flux could be monitored. After designing the assembly structure of the detector and relying on the CFBR-Ⅱ reactor, the 3DK2222A detector was calibrated in the flux range of 10¹²-10¹³cm^-2. The 3DG121C detector is calibrated in the flux range of 10¹³ cm^-2. The dispersion of the damage constant K of the detector is obtained as smaller or larger, the effective value and application method of the smaller dispersion are determined, and in cases of larger dispersion, the calibrated damage constant K can only be applied to the same detector, and the feasibility of the scheme is proved by high temperature annealing experiment.