In order to ascertain the hygrothermal environment of the middle hall building of Longju Temple, the temperature and relative humidity in building spaces were measured by a monitoring system based on Internet of Things technology. The spatial-temporal distribution of temperature and the relative humidity were analyzed. The characteristics of the hygrothermal environment were then evaluated. The influence on cultural relics was also considered. The results show that the building has good buffering and isolating effects against changes of external temperature and humidity, but that the internal hygrothermal environment is not conducive to the preservation of cultural relics due to high relative humidity and great fluctuation of humidity. Therefore, controlling humidity is the key to preventive conservation of cultural relics. The hygrothermal environment is the main cause of mildew on and other damage to the murals and it also threatens the structural safety of the bamboo-woven clay wall. These research results could enrich the hygrothermal environment understanding of ancient wooden buildings with bamboo-woven clay wall and provide a basis for understanding the causes of mural damage and wall structural problems.