The aerosol optical thinkness (AOT) during 2005～2010 over Beijing was investigated based on POLDER data and validated using AERONET monitoring data. The result was used to analyze the temporal variation and spatial distribution of AOT. The statistic result showed that high values were observed in southeast while low values were in northwest. The AOT decreased slowly year by year during 2005～2010. Monthly average AOT had significant differences, reaching the maximum 0.65 and the minimum 0.2 in July and November, respectively. The AOT in spring and summer was higher than that in autumn and winter, and it reached the maximum and minimum in summer and autumn respectively.