Aerosol optical properties are investigated using the data observed by sun-photometer CE318 from 2011 to 2014 at Beijing station. It shows that the aerosol optical depth in Beijing, China is high for the whole year, the annual means of aerosol optical depth (AOD) at 440 nm for the four years are about 0.67±0.70, 0.69±0.71, 0.73±0.66 and 0.75±0.66, respectively. The average monthly AOD shows similar seasonal variation with a maximum in spring and a minimum in autumn. According to aerosol classification, high aerosol level in Beijing is mainly caused by fine particles and there are also coarse dust particles in spring. Fine particles growth is found in four seasons, which is mainly a hygroscopic growth in summer and autumn and a coagulation growth in other seasons. Comparison of aerosol properties between dust and haze condition shows AOD in haze condition is generally higher than that in dust condition. The back trajectory analysis indicates that airmasses on the dust day are northwest wind across Mongolia grassland and Gobi deserts. The wind speed on the haze day is weak and dominated by southeast and northwest wind, and high aerosol load condition is caused by local accumulation and transportation from other places.