This paper theoretically analyses fin efficency of heat exchanger surface under frosting condition. From the compasrison of some fin efficiency functions, shortages have beenpointed out. On the basis of linearization of eothalpy of saturation humid air, a new fin efficiency function is developed. New function synthetically considers the effect of temperaturedistribution on fin surface and the heat transfer intensify of mass transfer so that it can moreaccurately reflect the fin effect to heat transfer performence. Analysis result shows that thefin efficiency is independent on air relative humidity in certain temperature range. This paper theoretically analyzes finning efficency of heat exchanger surface under frosting condition. From the compasrison of some fin efficiency functions, shortages have beenpointed out. On the basis of linearization of eothalpy of saturation humid air, a new fin efficiency function is developed. synthetically considers the effect of temperaturedistribution on fin surface and the heat transfer intensify of mass transfer so that it can moreaccurately reflect the fin effect to heat transfer performence. Analysis result shows that thefin efficiency is independent on air relative humidity in certain temperature range.