Air upglide motion on the frontal surface is the important cause for the frontal precipitation. That the mechanism of air upglide motion and downglide motion on the frontal surface is understood is necessary to forecast frontal precipitation. Based on the dynamics of atmosphere, the cause for air upglide motion and downglide motion on the frontal surface was studied. The glide motion of warm air on the frontal surface is related to the difference between the wind speed of warm air in the direction perpendicular to the frontal surface and the moving speed of the frontal surface. Warm air upglide occurs when the wind speed of warm air in the direction perpendicular to the frontal surface is less than the moving speed of frontal surface (uwc). Because the moving speed of frontal surface is less than the wind speed of cold air in the direction perpendicular to the frontal surface (c0 in the frontal zone; however, u/x<0 in the frontal zone is the necessary condition of upglide motion of warm air. Based on the dynamics of atmosphere, this article shows the formula of the frontal slope. Deduction of this formula works out the formula for analyzing u/x. The following conclusions can be drawn. First, air is upglide motion on the frontal surface when the frontal slope increases, on the contrary, downglide. Second, air is upglide motion when frontal intensity increases, on the contrary, downglide. Third, air is upglide motion when it flows from warm to cold; air is downglide motion when it flows from cold to warm. Fourth, air is upglide motion when the cyclonic vorticity develops on the frontal surface,on the contrary,downglide. Fifth, air is upglide motion when the speed of geostrophic wind in the direction perpendicular to the frontal surface is rising from lower to upper frontal surface,on the cotrary,air is downglide motion.