In order to study the structural feature of the atmosbhere boundary layer (ABL) over downtown of Beijing and the aerosol extinction characteristics in it during the Beijing Olympic Games, ABL over Beijing was observed with a MPL lidar in August, 2008. The vertical distribution of extinction properties and the height of ABL over the observation site were inverted. The observation data was compared with the results of observations in previous years at downtown of Beijing and Lihua Village in souther suburb of Beijing which was viewed as a standard of clean control site. The data indicate that the diurnal variation of ABL over Beijing during the Olympic Games is obvious, which is low in morning and evening, and high at noon time. The ABL height is relatively stable, it is always less than 2.0 km, its average height is 0.79 km. Compared with the data of previous years, the aerosol particle concentration level at downtown of Beijing during the Olympic Games is descended apparently. The aerosol concentration at observation site changes periodically every day.