A ground-based visible all-sky imaging system has been in operation for more than 2 years in Beijing (40°N, 116.5°E) since 2001. Using the 1428 images selected from those obtained from the year 2001 to 2003 we analyzed the spatial and temporal distribution of clouds and their seasonal variation in Beijing in two different ways. Some statistical results show that cloud distribution has some special character. We found that cloud fraction estimated from the all-sky image is relative to its height base-height and position relative to observer of cloud itself. To understand it further and to apply it to other researches of this system we simplified the cloud into two simple types: cube cloud and cylinder cloud for simulation. Then we studied the variation of the cloud fraction with its site on the principle of the equi-angle projection of the fish-eye lens. At the end of the paper the numerical simulation results are presented. Depending on different ratio of width to height cloud fraction has different variation. If ratio value is smaller than threshold value cloud fraction will increase at first and then decrease with the increase of the zenith angle.