By analyzing the relationship between midday soil heat flux (Gm) and apparent thermal inertia (ATI) by time series observations from YuCheng agroecological station, CAS, a method of estimating midday G by ATI was presented in the paper. ATI method is shown to have the close agreement with the observations. In situ observations from 2003 to 2005 were used to determine the coefficients of the formulation of Gm and ATI. Data in 2006 and 2008 were used to evaluate the method. In addition, effects of soil moisture on soil heat flux were specially investigated by field experiment. Soil water content is the dominant factor influencing soil heat flux for saturated or near-saturated soil (45%). As soil dries, soil temperature plays a more significant role and on the initial drying stage (from 45% to 30%), relatively higher soil temperature combined with high soil thermal conductivity make soil heat flux of wet soil much higher than that of dry soil.