Abstract: The horizontal ground heat exchangers(GHE) and particularly those deployed under pavement have gained increasing attention in recent years. One major advantage of the GHE under pavement is the significant amount of upfront cost that can be saved from drilling or excavation. In China, the potential for these GHE is immense due to the fact that there are more than 100, 000 km of infrastructural pavements being constructed each year. In this paper, we aim to study the thermal diffusion radius of horizontal GHE under pavement and the effect of air temperature fluctuation on the heat transfer capability. In order to achieve the research goal, we use the GeoCube thermal response test unit to conduct two in-situ thermal response tests(TRT), with 4 kW and 6 kW heating powers respectively on the GHE. In addition, a 3D numerical model is developed in COMSOL to simulate the heat transfer between the GHE and soil medium based on the experimental parameters of TRT with 4 kW heating power. The experimental result shows that the fluctuation in air temperature has a substantial adverse impact on the GHE′s heat transfer ability. Nonetheless, the adverse impact is weakened when the heating power of the TRT is increased. According to the experimental and numerical simulation results, the thermal diffusion radius of horizontal GHE under the pavement is found to be less than 0.75 m, thereby suggesting a minimum space to space distance between the GHE pipes to be 1.5 m to prevent thermal interference. The experimental validation also demonstrates that the numerical model can be utilized to simulate the heat transfer progress of a horizontal GHE under pavement and that the accuracy of simulation result is acceptable.