The atmospheric aerosol optical thickness and the precipitable water vapor provide the essential evaluation basis for the astronomical site selection, and important reference for astronomical observation. DTF type sun-photometer is used to observe the diurnal, seasonal variation of the aerosol optical thickness, ngstrm index and the precipitable water vapor in clear and cloudless weather from September 2013 to January 2015 in Delingha, Qinghai Province, China. The results show that the region of the aerosol optical thickness and the precipitable water vapor type can be divided into six kinds and has some seasonal characteristics. During different seasons, the aerosol optical thickness is the minimum in autumn, nearly in other seasons, and the precipitable water vapor is the largest in summer, obviously more than other seasons. The ngstrm index is the minimum in spring, the largest in winter and nearly in summer and autumn. Overall the small aerosol optical thickness, the low amounts of precipitable water vapor, and the clean air make Delingha an appropriate site for astronomical observation.