During the long-period exposure experiment, the dark current noise of scientific grade change coupled device (CCD) is one of the major noises, the relation of average dark current which is 2.43 ADU/(s·pixel) and 0.4854 ADU/(s·pixel) at -10 ℃ and -20 ℃ and exposure time are tested, respectively. The relation of dark current and temperature is tested. The result shows that the dark current varies with temperature exponentially. As the temporal response characteristics of mechanical shutter have a direct effect on the short time exposure of the CCD, the experiment also shows the relation of exposure time and the average counting of CCD. The result indicates that the mechanical shutter can be fully opened at 18 ms.