A method of retrieving aerosol optical depth (AOD) is developed using daily horizontal broadband direct solar radiation, including an “equivalent” instantaneous solar zenith angle model. Furthermore, a cloud-contaminated reduction method, which is based on aerosol scaling height, is proposed to calculate monthly mean AOD. Numerical simulation and error analysis show that: 1) The mean value of AOD retrieving errors due to instability of the “equivalent” instantaneous solar zenith angle model is 3.66%; 2) AOD diurnal variation has little influence on the mean value of AOD for a relatively long time; 3) the AOD retrieving error is less than or equal to 4％ when the error of diffuse solar radiation is less than or equal to 20％. A comparison between AODs retrieved from this method and the AERONET shows that the AOD retrieving method and the cloud-contaminated reduction method are both effective. The correlation coefficient and mean bias error between AODs from this method and the AERONET at 0.75 μm in clear days are 0.95 and 0.02, respectively. With the cloud-contaminated reduction method, the seasonal and yearly mean AODs show a good consistent with those from AERONET.