Abstract: The atmospheric turbulence is a major problem in the optical astronomy as it drastically reduces the angular resolution of telescopes.The degradation of the astronomical image quality is also caused by the atmospheric turbulence.The distortion of the image as well as its motion may vary considerably with time and with the location of the observer.These random displacements are called as the astronomical seeing.The degradation of the astronomical image quality can be characterized by the seeing parameter r₀. The relation between the measured r₀ and the video CCD exposure time in the image motion method measuring the seeing parameter r₀ is given in this paper.It is indicated that the exposure time must be shorter than the atmospheric coherent time.If the exposure time is longer than the coherent time,one will obtaine a bigger r₀ value.For this and the need to check the instrument,an accurate measurement of the video CCD exposure time is necessary.In this paper, an experiment we designed to measure the exposure time is presented.The result of the experiment shows that this method is reliable.