The electron irradiation effect of a 4H-SiC npn bipolar transistor UV detector is investigated in this paper. When the phototransistor is biased at 5 V, before irradiation, its dark current is about 58 nA, and its responsivity to 365 nm UV light is about 31A/W. After the device is irradiated by a 10 MeV e-beam, the order of magnitude of the dark current decreases to 10^-11 A, and the responsivity decreases to about 1/8 of the original one. After irradiation, the responsivity of the device is significantly affected by the bias voltage: it decreases as the bias voltage decreases, and when the phototransistor is biased at 3 V, the responsivity decreases to 2.25 A/W. E-beam irradiation also affects the switching response of the UV detector, which results in a longer total time of response. In this paper, the circuit model of phototransistor operation is established, and the decrease of light generation current, the decrease of transistor gain and the increase of series resistance caused by electron beam irradiation are the main reasons for the degradation of photodetector’s UV response performance.