Abstract: The thermal insulation performance of thermal protection materials or structures is an indispensable and important factor in the safety and reliability design of spacecraft and high speed flight vehicles. To obtain the temperature difference of the radiating surface for plate specimens under three different boundary conditions in heat insulation experiments (with the specimens placed either vertically or horizontally with respect to the radiating surface facing down or horizontally with respect to the radiating surface facing up), three thermal test setups are established to test the thermal insulation performance of light-weight ceramic specimens at different temperatures (with the radiating surface open). It is shown that the radiating surface temperature is the highest when the specimen is placed horizontally with the radiating surface facing down, while it is the lowest when the specimen is placed horizontally with respect to the radiating surface facing up. When the heating surface temperature is 1000 ℃ and the time is 1800 s, the radiating surface temperature of the horizontally placed specimen with the radiating surface facing down was 19.7% higher than that of the specimen horizontally placed with respect to the radiating surface facing up; The radiating surface temperature of the vertically placed specimen is 2.3% lower than that of the horizontally placed specimen with respect to the radiating surface facing down. In addition, the numerical calculation results agree very well with the experimental ones, confirming the credibility and the accuracy of the experimental results. The results provide an important reference for the design of thermal protection systems and determination of the experiment scheme for spacecraft and hypersonic vehicles