This study aims to estimate the thermal performance of a horizontal spiral-coil ground heat exchanger (GHE), focusing on the geometry factor of the spiral-coil. Prior to the thermal performance evaluation, an indoor thermal response test (TRT) was conducted with a 5 m x 1 m x 1 m steel box filled with dry Joomunjin sand and TRT equipment was used to validate the numerical analysis using the commercial program COMSOL Multiphysics 5.2a, which is based on computational fluid dynamics (CFD). Next, the number of analysis cases was set by selecting the key factors affecting the thermal performance of a horizontal GHE: installation depth, fluid velocity, thermal conductivity of the ground, and numerical studies were conducted on the 16 cases. As a result of the analysis, it was observed that the pitch does not affect thermal performance if the pitch exceeds 0.6 m. These same results were derived even when additional changes in the key factors were taken into consideration. In addition, it was revealed that the spiral diameter does not affect the thermal performance of the horizontal spiral-coil GHE. Therefore, we concluded that the geometric factor that determines the optimal thermal performance of the horizontal spiral-coil GHE is not the spiral diameter but the pitch, and the 0.6 m pitch of the horizontal spiral-coil GHE can provide a compromise between the thermal performance and installation land space.