CO2 concentration in cave air is one of major factors controlling the precipitation and erosion of carbonate in caves, especially in show caves. In order to understand the spatial and diurnal variation and the possible controlling factors, CO2 and temperature as well as relative humidity in cave air are observed by portable CO2 meter linked with auto data logger and auto temperature as well as RH data logger respectively in the Benxi Water Cave, Liaoning, China. The results show that the spatial distribution of CO2 concentration in the cave’s air can be divided into two situations. Firstly, CO2 increases abruptly from 500 ppm at the cave entrance to 4 500 ppm at 370 m deep of the cave; secondly, the CO2 concentration keeps stable with small regimes between 4 500 and 4 800 ppm. Compared with data observed in other seasons, the distance in the first situation differs from one season to another. This may be controlled by seasonal change in temperature led by the exchange of airs between the outside atmosphere and the inner cave air, and also may be modulated by the human activity. The mean values of CO2 in cave air, in the second situation, decreases slowly during observing period, which corresponding to the deceasing trend of visitor number and indicating that visitor number makes important effect on diurnal variation of CO2 concentration in cave air. The slight variation of CO2 concentration in the second situation maybe relates to the change of volume of cave chamber or the distribution of fractures and fissures. The auto data logger set at 250 m away from entrance records two CO2 concentration peaks every day with one peak at noon and another at mid night more or less. By comparing with the variation of air temperature both inside and outside the cave, this double peak pattern possibly contributes both to respiration of people and exchange of air in cave with atmospheric air.