The heat balance equation of an element of ground surface including the term of latent heat exchange was presented. The method of temperature linearization was used.By selecting the absorptivity of ground surface and the reciprocal of heat exchange resistance between ground surface and atmosphere as random noncorrelated variables,and assuming that the two variables obey normal distribution,the expression of the probability density of temperature difference between ground surface and constant temperature underground was derived.The average temperature of ground surface was calculated.Compared with the data measured,the result is satisfactory.