Temperature and humidity are two of the most important factors that influence the preservation of cultural relics. It took seven months to get continuous test data from an airtight showcase in Emperor Qinshihuang’s Mausoleum Site Museum. The results show that the internal temperatures of the showcase fluctuated with the change of external temperatures and that the stability of humidity depends on the air airtightness and difference in relative humidity between inside and outside of the showcase. Good airtightness could effectively reduce interference of ambient humidity. A short-term fluctuation of internal relative humidity of an airtight space was mainly affected by environment temperatures, so good control of temperature could effectively reduce the fluctuation of relative humidity.