Shallow geothermal energy is defined in The Standard for Evaluation of Shallow Geothermal Energy as the heat energy which is "contained in the surface depth no more than 200 meters and not exceeds 25℃". In addition, there is no specific rule on the Borehole Heat Exchanger(BHE) depth in Technical Code for Ground-Source Heat Pump System. Generally take 100-120 meters, according to the actual engineering experience and calculation. A three-dimensional numerical heat transfer model is proposed in this paper,which calculates the heat transfer in different depths and analyzes the influence of various factors on the depth of BHE. Conclused that the flow rate in the pipe is the most important factor. Geothermal gradient, thermal physical properties of backfill material and the spacing between pipes also have influence on the heat transfer of the BHE in different depths.So in actual project,if we want increase BHE depth, we should increase the flow rate in the pipe and reasonablely increase BHE size.