In order to restrain the effect on the images from the variation of the bias caused by factors such as the working temperature and the aging of electronic devices in video processing circuit of CCD camera, the bias of the circuit are modified in real time. Firstly, the time sequence of the modules in the drive circuit of CCD camera and the image processing circuit was analyzed and modified to acquire the bias variation of the every frame image as passing the circuit. Then the modified values of the pixel of every image were calculated by the image processing block. Finally, the modified image was output. Experimental results indicate that the bias of each image is on the same level after being modified. The mean of modified frame is some 107 DN under test mode. The difference of mean between two output port images is reduced from 170.9DN to 4.8DN. The method not only eliminates the changes of video processing circuit’s bias, but also removes the difference of bias between the two output port of CCD.