A measurement system of temperature field under vacuum condition is established based on fiber Bragg grating (FBG). A 304 stainless steel frame, which is similar to the support structure of the vacuum chamber in Tokamak devices, is used as the research object to conduct steady-state thermal analysis under both vacuum and normal pressure conditions. Specifically, an aluminum alloy heating plate is adopted to heat the stainless steel frame under either vacuum and normal pressure conditions. As packaged with micro steel capillary, four FBG temperature sensors are placed into measurement points to detect temperature. The experimental results show that the temperatures of four measurement points under vacuum condition are higher than those under normal pressure condition. Meanwhile, the temperature of four measurement points reach stability costs 4900 s and 6150 s, respectively, under vacuum and normal pressure conditions. It is shown that heat transmits by the way of both thermal conduction and thermal radiation under vacuum and normal pressure conditions. As the stainless steel frame contacts with air under normal pressure condition, which leads to thermal losses via thermal convection, the heat loss of stainless steel frame is smaller, and the heat transmission efficiency is higher under vacuum condition.