A simplified heat transfer model is proposed for conveniently determining the thermal properties of deep ground soil at worksites and other associated engineering applications. The model does not need exact information of the U-tube location, spacing between up and down tubes, the properties of the tube and the backfill, and other parameters in the borehole as usual cases and greatly eliminates the errors caused by measuring the parameters mentioned. A practical site test was conducted to obtain heat flux imposed on the buried loop, water flow rate, and temperatures at the outlet and inlet of the loop by an in-situ test apparatus. The simplified model, together with optimal estimation algorithm, was used to determine the thermal conductivity of the deep ground soil in the worksite.