By using the direct solar radiation data measured by a CE-318 automatic sun tracking spectrophotometer,the optical depth of atmospheric aerosols is calculated by using the Bouguer theorem and its variational characteristics in four seasons in Yinchuan area are analyzed.The results show that the optical depth of atmospheric aerosols shows apparently daily and seasonally variations.There are four patterns of daily evolution(0900-1600) BST,and they are:1)the diurnal evolution of the optical depth of atmospheric aerosols are relatively steady;2)the diurnal evolution of the optical depth of atmospheric aerosols shows a general trend of increase;3)the optical depth of atmospheric aerosols exhibits a peak value at noon;4)the optical depth of atmospheric aerosols shows a peak value in the morning but its variation is small at other time.The seasonal variation of the optical depth is larger in winter and spring,smaller in autumn and the smallest in summer.The main component of atmospheric aerosols is dust in spring,and the particles emitted by human beings in winter.As far as in summer, the optical depth and number concentration of atmospheric aerosols are relatively small because of the abundance of precipitation and the plenty of water vapor.