In the in-situ test of ground thermal properties for vertical tubes, simplified analysis method is frequently applied to calculating the thermophysical parameters of rock and soil mass. Integrated thermal conductivity and borehole thermal resistance are worked out then. This method may result in different levels of error under variable test conditions. In this paper,based on the heat transfer numerical method,the authors studied the main factors that influence the thermal conductivity and the borehole thermal resistance accuracy, such as circulation medium flow rate, loading power, interval between two tests, performance of test equipment, depth of test holes, thermal conductivity of backfill material, seepage in rock and soil mass, and geothermal gradient. Based on the calculation results of heat transfer, the authors put forward methods for reducing the calculation error of linear heat source:Selecting relatively reasonable test parameters according to the test conditions, eliminating the system error by the model experience correction method or using improved numerical models.