Abstract: Aiming at the problem that the irradiation light output of CsI(Tl) scintillator detector changes constantly during the long time experiment, with the stability control of dark field signal, a pebble flow detection system with a 5∶1 ratio reduction of the actual core was irradiated for 48 h with no-load γ ray. Besides, the controlled result of dark field stability and the irradiation light output of the detector were recorded. Experiment results show that after the γ irradiation, the irradiation light output of the scintillator detector increases firstly, and then decreases linearly after it reaches a maximum value. After the analysis of irradiation light output, it is found that the irradiation light output rise ratio and the corresponding irradiation damage between detectors are different despite of the same geometric and physical component. However, the cumulative dose corresponded to the maximum irradiation light output is basically the same, and it has little relevance with the size of the initial irradiation light output. Based on above, a corresponded exponential method and a linearity method were proposed respectively for the correction of the rising and falling stages of the irradiation light output, and it provides a solution for the correction of the projection data during the process of image reconstruction in PB-HTGR pebble flow detection system.