Thermal conductivity obtained via thermal response test (TRT) is one of the essential parameters for the design of shallow geothermal systems, especially a borehole heat exchanger (BHE). During TRT analysis, several factors (e.g., start time of analysis and test duration) could influence the estimated thermal conductivity. In addition, the influence of the factors may be different depending on the characteristics of the borehole and ground. This study investigated the effects of the start time and test duration through numerically generated TRT data under a diversity of environment and BHE information such as the thermal conductivity of grouting and ground. The generated numerical data were analyzed with three analytical approaches having different assumptions about the heat source to interpret the sensitivity of factors in the TRT analysis. By conducting the analytical sensitivity analysis, the importance of determining the appropriate start time and test duration could be emphasized when designing the test.