Abstract: In order to understand the convective heat transfer of the horizontal flat at different pressures, a sealed cabin that can provide different pressures and temperatures is designed, and the thermal performance of the horizontal flat in several fixed heating input(75 W/m², 150 W/m², 300 W/m2) under different pressures (0.1 Pa, 0.1 kPa, 0.2 kPa, 0.5 kPa,1 kPa, 10 kPa, 50 kPa, ambient pressure) is experimentally investigated. By comparing the radiant heat exchange with the natural convection heat transfer, the convective heat transfer coefficient under different pressures is obtained. The experimental results show that the convective heat transfer coefficient is very small when the absolute environmental pressure is less than 1 kPa; it might be meaningful only when the absolute pressure is more than 1 kPa; and it increases quadratically along with the increase of the environmental pressure.