The temperature uniformity of thermocouple calibration furnace plays an important role in the calibration of base metal thermocouple and is an important parameter to evaluate the accuracy of thermocouple calibration. In JJF 1637-2017 “Calibration Specification for Base Metal Thermocouples”, it is proposed to use thermostatic block to improve the uniformity of thermocouple calibration furnace. Many scholars verify the feasibility of using thermostatic block through experimental methods, but experimental research requires many built-in standard thermocouples which are not completely consistent with the actual working state and the temperature data can be obtained is limited. In order to further explore the heat transfer mechanism of improving temperature uniformity by using thermostatic block, the temperature distribution of temperature field in furnace is calculated by theoretical analysis and numerical simulation. The calculation results show that the alloy thermostatic block plays a good role in optimizing the temperature field because of its good thermal conductivity. Compared with not placing the thermostatic block, the maximum temperature difference in the axial direction is reduced by 2 times and the maximum temperature difference in the radial direction is reduced by 1 times, which is an important means to improve the temperature uniformity.