The effect of electron irradiation on CMOS image sensor was studied, focusing on average dark current and light intensity response. The CMOS image sensors, some in operating mode and the others in non-operating mode, were radiated simultaneously. The data were collected when the total dose of radiation reached 5×103 rad, 1×104 rad, 7×104 rad, 1×105 rad, 5×105 rad, respectively. The results showed that, when the total dose of radiation exceed a threshold between 7×104 rad and 1×105 rad, dark current of the imager showed a basically linear relationship with the total dose of radiation. When concerning the light intensity response, for the imagers which were radiated in non-operating mode, radiation have no effect on the response; while for the imagers which were radiated in operating mode, with the total dose of radiation exceed a threshold, 7×104 rad, the response curves shifted down and the slopes decreased with the total dose of radiation increased. In other words, radiation reduced sensitivity of the imagers. At last, the model of dark current versus total dose of electron radiation was developed after theoretical analysis. It is shown that after working in the space for a long time, CMOS image sensors are easily affected by the total dose effect of radiation, thus reasonable protection measures against radiation are needed.