The diumal observations on atmospheric CO2 dynamics in the Guilin karst depression ecosys- tem indicate the following: the karsi depression has a certain regulation effect on atmospherical CO2 and may accelerate karst development; the photosynthesis- respiration of bush vegetation is the direct factor for controlling the atmospherical CO2 dynamics in the karst depression ecosystem , while the CO2 emitting from soil, is less important. CO2 is both the important driving force of the karst process and the focus of the global change research. The key factor for carbonate rock dissolution and uptaking atmospheric CO2 is the dynamics of CO2 and water and their actions near the surface. The results of this research will be conductive to clarifying the mechanism of carbonate rock dissolution and uptaking atmospheric CO2 in terms of CO2 dynamic change and provide a new clue for the karst dynamic system and world carbon cycle study.