Abstract: The energy pile technology is a new energy-saving technology that integrates the functions of ground source heat pump and building pile foundation. In order to study the thermo-mechanical characteristics of energy piles and their effects on the other parts of pile foundation, field tests on the thermal response of a 2×2 pile-cap foundation under single pile heating conditions are carried out. The temperature and strain changes of energy piles, diagonal piles and cap are measured. The mechanical properties of the energy piles due to temperature changes, the influence laws on the soil around the pile and the structure of the cap are discussed. It is shown that the maximum constraint compressive stress of 3.94 MPa is generated in the middle of the pile during the summer operation of the energy piles. The condition of the thermal stresses associated with the complete heating and restraint of the pile provides a suitable upper bound for design, and the measured value is about 48% of the upper bound. Under the combined effects of atmospheric temperature and operating pile, an additional tensile stress of approximately 1.05 MPa (approximately 43.8% of the tensile strength of concrete) will be induced in the middle of the cap. The head displacement of the energy pile is about -0.6 mm (0.6‰ of pile diameter) under the thermo-mechanical coupling.