The steam generator in a sodium-cooled fast reactor is an important part of the nuclear reactor system and is a key hub for transferring heat from the reactor's second loop to the third loop. During the start-up of the steam generator from cold, the steam generator needs to be preheated to the target temperature first. During this preheating process, the temperature of the steam generator itself needs to be monitored in real time, and this temperature is used as an important control signal for the operation of the electric heater. Currently, temperature measurement is usually performed by arranging thermocouples on the external surface of the steam generator. However, a bad thermocouple arrangement will make the measurement results deviate from the actual temperature. In this paper, the CFD software Fluent is used to explore the temperature measurement effect of different thermocouple arrangement, and carried out an experimental study of the measurement effect of four different thermocouple arrangement. The four mounting methods are: inserting the thermocouple inside the equipment body as a calibrated temperature value, mounting the thermocouple vertically on the surface of the equipment by means of a sleeve, spot-welding the thermocouple to a stainless steel plate by means of a stainless steel sheet, and spot-welding the thermocouple directly to the equipment body by means of a stainless steel sheet. The results of the study show that the addition of an isothermal block between the steam generator and the thermocouple helps to reduce the impact of temperature measurement inaccuracies caused by factors such as gaps between the thermocouple and the surface of the equipment, and such optimisation is significant for the safe operation of the equipment. At the same time, the research in this paper for the same type of high-temperature equipment outside the surface temperature measurement of the thermocouple arrangement optimisation provides a good reference.