A same cloud has different view-angles when it moves from overhead toward the near-horizon.In this paper,we simulate the track of the cloud's movement based on the principle of the equi-angle projection of all-sky imager(fish-eye lens) and estimate the cloud fraction during its movement.The cloud is simplified as the cubic and cylindric objects of two different sizes in the simulation.To get the characteristics of variations of the cloud fraction,we calculate the cloud amount and the ratio of the cloud's frank-fraction to the cloud-fraction during the movement of the cloud.Results show that the variation of the cloud-fraction is related to the ratio of the cloud's base to height(RBTH).When the RBTH is larger than a threshold,the cloud-fraction will increase at first and then decrease with the zenith angle's increase.Otherwise,it will decrease all the time.The degree of the decrease is related to some parameters such as the height,RBTH and size of the cloud.And the frank-fraction will account for more and more portion of the whole cloud cover when the zenith angle increases.