In order to verify the performance of space deployable structures before launch, separation and deployment tests should be carried out in large space environment simulation chamber, in which a simulated on-orbit environment can be achieved. In separation and deployment tests, high-speed camera system is often employed to capture the test image. The high-speed camera system is also exposed to the cryogenic and vacuum environment in large space environment simulation chamber. Consequently, cryogenic and vacuum adaptability of the system should be very strong to withstand the extreme environment in space environment simulation chamber. A common solution is using optical chamber. The equipment was set in a pressurized chamber which can insulate inner environment from the outer vacuum environment, and the gas in the chamber can also help to heat convection, therefore heat generated by the device can be transferred to the chamber wall. This paper aims to explore the thermal design of optical chamber for high-speed camera or other special equipment used in extreme environment. Using rule number analysis method, Heat transfer model in vacuum and ambient environment had been built to analyze the characteristics of heat transfer under different thermal power, characteristic length, and environment. recommended limits were delivered by the rule number analysis. CFD method was applied to verify the rule number analysis above. The two methods above reached a similar result, which demonstrated the effectiveness of the analysis.