Atmospheric aerosols regulate the balance of radiation budgets of the surface and atmosphere through direct,indirect and semi-direct effects,which affect regional and global climate change. In order to evaluate the aerosol radiation effects accurately,it is necessary to have a good understanding of aerosol optical and microphysical characteristics. At present,there is no sufficient understanding of the aerosol optical and microphysical characteristics over the typical semi-arid region in Loess Plateau. Based on the 2006-07-28 to 2012-08-10 sun photometer data from the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL),dust aerosol was determined by using a strict judgment,and then dust aerosol optical and microphysical characteristics were analyzed. The work has important scientific significance and application value. The main results are as follows: Spring has the highest (2.80) dust aerosol optical depth (AOD),which is mainly distributed from 0.3 to 0.8 (about 82%),with a daily average of 0.630.44,indicating that the largest number of dust days occur in spring. Angstrom exponent with maximum and minimum in spring (0.002) and autumn (0.525), which is opposite in phase to AOD,ranges between 0.2~0.4 (about 56%) and has a daily average of 0.27 0.12,showing that dust aerosols are mainly large particle size. The coarse mode dominates the size distribution of the dust aerosol,and the spring concentration is the largest. Both total and coarse mode volume concentrations have similar annual variation to AOD and reach their maximum in April with the fine mode volume concentration changing very little. The aerosol total and coarse mode effective radius ranges from its maximum in spring to its minimum in autumn,and the amplitude of the fine mode particles is small. The real part of the refractive index with the change of the effective radius is higher in spring and winter while it's lower in summer and autumn,which indicates that dust aerosol has strong scattering. In opposite phase to imaginary part of refraction index,the single scattering albedo is larger in spring,but has a minimum value in February,so dust aerosol absorption is weaker than black carbon aerosol. The asymmetry factor reaches its minimum in spring,which indicates that dust aerosol has stronger forward scattering than other kinds of aerosols.