Abstract: The formulation and characteristics of cold-air drainage are studied in this paper. The terrain is considered as an ideal two dimensional slope with infinitely extended in Y-direction. At general situation, the varied regulary of coldair drainage is obtained through solved numerically the atmospheric movement equations, the thermodynamic equation, the surface thermal balance equation and the heat conducted in the soil equation. Finally we test the variety of cold-air drainage with the amount of cloud, the coefficience of turbulent exchange, the angle and the length of the slope, the heat flux in the soil and the thermal characteristics of the soil.