This article presents a quantitative analysis of the effects of sand-bentonite backfill materials on the thermal performance of borehole heat exchangers (BHEs). Laboratory thermal probe tests were conducted to measure the thermal conductivity of sand-bentonite mixtures under different mixed ratios. Based on microscopic observations, the mechanism of bentonite affecting heat conduction between the sand grains was analyzed. Then field tests were carried out to compare the thermal performance of two double U-shaped BHEs with different backfill materials. Test results showed that the thermal conductivity of sand-bentonite mixtures first increased with increasing percentage of bentonite by dry mass, then reached a peak at the range from 10% to 12%, beyond which the thermal conductivity decreased quickly. For the BHE with an optimal sand-bentonite backfill material, the heat injection and heat extraction rate were enhanced on average by 31.1% and 22.2%, respectively, compared with the case with a common sand-clay material. These results can provide helpful guide for the design of ground source heat pump systems.