At present, geothermal energy is a promising research area but still has numerous areas to be explored. The thermal performance verification of U-shaped closed geothermal heat exchange was proposed to improve the effective thermal conductivity value applying initial ignoring time (IIT). To verify the thermal performance of the underground heat exchanger in the rock layer according to the surrounding environment and groundwater conditions, verifications were conducted for the amount of circulating water, amount of heat input, and groundwater conditions. The circulating fluid flow rate of the system, supply of water into the ground, and influence of the change of supplied heat source were analyzed. The effective heat conduction characteristics were analyzed using the linear heat source model and initial exclusion time. Therefore, in the thermal response test, the main error occurs according to the determined initial removal time, and the reliability of the effective thermal conductivity value is increased by applying IIT to reduce this error.