The coaxial thermocouple has a great application prospect in the field of heat flow measurement in explosion field because of its excellent anti-erosion performance and measuring ability in complex and harsh environment. The thermal product coefficient is one of the important parameters of the measurement accuracy of coaxial thermocouple, and its value will change with the nature and type of sensor material and the process difference in the preparation process. Therefore, it is necessary to calibrate the thermal product coefficient of the prepared sensor. At present, the surface joint of K-type coaxial thermocouple is mainly formed by blade scratches, diamond grinding rod and different mesh sandpaper, and different joint formation methods have an effect on the thermal product coefficient. At present, there is no research on the influence of sandpaper with different mesh numbers from 200 mesh to 800 mesh. The thermal product coefficient of the sensor prepared by sandpaper with different mesh numbers, single blade scratch and single diamond grinding rod was tested to study its influence rule. The experimental results show that the thermal product coefficient of the coaxial thermocouple with unidirectional scratch and grinding is closer to the value of the thermal product coefficient of the material in this direction, and the value of the sensor prepared by sandpaper will increase with the increase of the number of sand paper mesh, and the thermal product coefficient of the sensor prepared by 200 mesh is about 20% lower than that prepared by 800 mesh. At the same time, through numerical simulation, the transverse heat transfer will cause overheating of hot joints, and the influence is reduced by installing stainless steel bushing. The experimental results show that the difference between the presence and absence of bushing is about 1%-2%. The conclusion of this study can provide theoretical basis for the structural optimization of K-type axial thermocouple sensors.