Abstract: It is easy to reduce the service life and heating and cooling efficiency of ground source heat pump system by selecting the inappropriate type of ground heat exchanger. Taking two ground source heat pump projects in Xi'an as the engineering background, based on the constant heat flux line heat source model and the slope method, the influence of sand-clay proportion and pipe diameter on the heat transfer performance of double U-shaped buried tube heat exchanger(DUBTHE) was analyzed through field geotechnical thermal response test(TRT). The results show that the initial average temperature and comprehensive thermal conductivity of rock and soil are 17.70 ℃, 1.54 W/(m·K) and 18.83 ℃, 1.79 W/(m·K) respectively, the proportion of sand layer is 16.71 % and 45.99 %. The comprehensive thermal conductivity of geotechnical soil calculated by the weighting method is 1.15 to 1.25 times of that measured by TRT. When the percentage of sand layer in-creased from 16.71 % to 45.99 %, the initial average temperature and comprehensive thermal con-ductivity of the geotechnical soil increased by 6.38 % and 16.23 %. And with the increase of pipe diameter from 25 mm to 32 mm, the heat exchange per linear meter increased by 12.76 % and 35.61 % in summer and winter, respectively. When the proportion of sand layer is less than 45 % and the cooling and heating load of ground source heat pump system is close, DN25 DUBTHE should be adopted. when the proportion of sand layer is more than 45% and the heating load of ground source heat pump system is higher than the cooling load, DN32 DUBTHE should be adopt-ed. It can provide some reference significance for the design of ground source heat pump system under similar sand-clay proportion conditions.