Abstract: The thermal conductivity of soil at different depths is a key parameter for the evaluation of shallow geothermal energy. A thermal response test based on the actively heated optical cable (ATRT) is one of the effective ways to obtain the in-situ distributed thermal conductivity. Compared with traditional thermal response test (TRT) and distributed thermal response test (DTRT), ATRT has better test efficiency. One of the key issues that determines the error and the effect of ATRT is how to effectively lay a heat source that satisfies the assumption in the line source model. Using the finite element numerical simulation software ComsolMultiphysics, a two-dimensional finite element heat transfer model of porous media was established, and we explored the influence of U-shaped internal heating cable's spacing on the calculated thermal conductivity. The results show that the thermal response process of ATRT can be divided into three stages, namely the cable-influence stage, the grout-influence stage, and the soil-influence stage. U-shaped laying of the cable will increase the heating time when the soil influence stage is stable, which is not conducive to improve the efficiency of ATRT. Due to the influence of simplified calculation, the thermal conductivity calculated from the test results of the U-shaped optical cable has a peak value significantly higher than the thermal conductivity of the soil at the initial stage of heating. With the increase of heating duration, the temperature rise rate under the condition of U-shaped optical cable layout gradually approaches the ideal line heat source temperature rise rate, the calculation result is closer to the true value, and reducing the spacing can speed up this process. As the spacing increases, the recommended heating duration will increase rapidly in order to control the error within 10%. Therefore, to improve the test efficiency, the distance between the U-shaped optical cables should be minimized in the field.