The quality of flow temperature data in the low-speed wind tunnel is the key to the character of the flow field of the wind tunnel. This article carries out special research on the compensation method of thermal conductivity error and radiation error in thermocouple measurement. Based on the basic principles of heat transfer, research is implemented on the method of the thermal conductivity error compensation with the fixed value of the immersed length of the thermocouple wire and the method of the radiation error compensation with the fixed value of relative change rate of the surrounding temperature. Simulation results of the quantitative relationship between the immersed length of the thermocouple, the relative change rate of the surrounding temperature, and temperature error are obtained. To evaluate the error mechanism analysis and simulation results, a wind tunnel validation test is carried out. Test results show that the measured thermocouple immersion error results are consistent with the theoretical simulation results, and the relative change rate of surrounding temperature ω<1 can significantly reduce the influence of temperature measurement radiation error, which can reduce the influence of temperature measurement radiation error on temperature uniformity by 7. 12 times. The method presented in this article provides useful exploration and technical basis for the engineering application of the thermocouple in gas medium.