Saltpeter in karst caves is abundant and widely distributed, historically serving as the primary source for gunpowder production and playing an important role in ancient warfare and societal development. Despite the long exploitation history of saltpeter in karst caves, research remains limited, and the metallogenic mechanisms are poorly understood. In this study, field geological investigations and nitrogen and oxygen isotope analyses were conducted on actively forming saltpeter in the Middle Cave of Dashiwei Tiankeng, Guangxi. We found that the lower section of the Middle Cave is connected to an underground river, with numerous jet holes linking it to the lower cave on both the floor and walls. White saltpeter powder is concentrated around these jet holes, nitrate and other aerosols are being emitted from these openings. The nitrogen and oxygen isotopes of the saltpeter exhibit typical characteristics of nitrate formed through ammonium nitration in soil. It is suggested that the source of these nitrates is underground river water. Certain soluble salt components, such as nitrate, in the river water evaporate into water vapor and migrate through the cave in gaseous form. The saltpeter cave, comprising a series of large chambers and narrow tunnels, functions as a natural gas compression refrigeration system, playing a crucial role in the formation of cave saltpeter. When airflow within the cave transitioned from a large chamber to a narrow tunnel, the pressure increases, the flow velocity accelerates and the temperature rises. Conversely, when airflow moves from a narrow tunnel into a large chamber, the gas temperature drops rapidly due to sudden pressure release, reducing the solubility of nitrate and other salt components in water vapor. This leads to gradual super-saturation and subsequent precipitation, forming salt aerosol deposits. The sequence of salt saturation precipitation follows an inverse relationship with solubility. Sulfates, which have the lowest solubility, precipitate and settle first, concentrating in the lower cave halls of the cave system. Subsequently, chlorides, with higher solubility, accumulate in the middle cave halls, while nitrates, possessing the highest solubility, ultimately gather in the upper cave halls or cave mouths. The greater the distance between the underground river and the upper cave opening, and the more chambers present, the more efficient the separation of different salts, leading to the formation of higher-grade saltpeter. Furthermore, caves with longer accumulation periods tend to develop larger-scale saltpeter deposits. When the tunnel becomes completely obstructed, saltpeter mineralization terminates.