CO₂ is an important driving force in karstification. Studying the variation characteristics of CO₂ in each zone of a karst cave system during different seasons might contribute to revealing its migration path and to further exploring CO₂ sources in caves. To this end, we monitored the soil, cave drip water, and cave air CO₂ in Mahuang Cave, SW China. We analyzed the vertical migration characteristics of CO₂ based on the carbonic acid equilibrium system theory and the stable isotopes of carbon through statistical analysis. The results show the following: (1) There are obvious seasonal changes in CO₂ in each zone of Mahuang Cave, generally indicating high rainy season and low dry season characteristics. (2) The CO₂ in Mahuang Cave is mainly supplied by the soil, bedrock, and the external atmosphere. The rainy season has plentiful rainfall, and the CO₂ in the cave is mainly derived from soil CO₂. In the dry season, with lower temperature and little rain, cave CO₂ is mainly attributed to the external atmosphere, brought by the ventilation effect, and carbon in the bedrock. (3) Carbonate precipitation in Mahuang Cave is dominant throughout the year. The seepage migration process and carbonate precipitation are controlled by the soil “CO₂ effect” and drip water type.