Abstract: On the basis of introducing the basic categories of atmospheric refraction and their existing condition, the forming processes of three kinds of atmospheric duct are expounded.Several main characteristics of atmospheric duct are summarized and analyzed, and field sounding data from the Western North-Pacific Cloud-radiation Experiment (WENPEX) and meteorological data around the Xisha sea area are used to validate these characteristics. Meanwhile the sensitivities of the evaporation duct height to the variations of atmospheric humidity, the air-sea temperature difference, and horizontal wind speed are examined. With the analysis of the effect of atmospheric duct on the propagation of electromagnetic wave, the maximum trapped-wavelength and the critical emitting angle of elevation for electromagnetic wave which can form sduct propagation are derived. At the same time the four kinds of necessary condition for electromagnetic wave to form duct propagation are brought forward. The effects of atmospheric duct on ultrashort wave propagation, radar observation, short wave communication etc. are also discussed.