The enrichment of limestone gas in the roof of the No. 8 coal seam of the Taiyuan Formation in the Shenzhou Coal Mine on the eastern edge of the Ordos Basin, north-central China, is a special discovery of the near-source coal measure gas reservoir type. Studying the origin type and generating mechanism of limestone gas is helpful for gas control and broadening the research field of coal measure gas. The origin type of limestone gas is identified by gas isotope tests. Based on the limestone water chemistry and hydrogen and oxygen isotope tests, the generating mechanism of the examined limestone gas is expounded upon by combining the lithology characteristics and structure and hydrogeological conditions of the coal measure strata. The main component of L-1 limestone is calcite, with a very low organic content. The L-1 limestone has no means of producing organic hydrocarbon, and the adsorption capacity to methane is extremely small. The coal measure gas is a secondary biogenic gas, and the gas or gas generation matter is derived from No. 8 and 10 coal seams. The limestone water is replenished by atmospheric precipitation in the west and flows along the inclined strata from the west to the S-1 syncline axis in the mine field. The enclosed environment of the L-1 limestone layer provides a reducing environment for the microorganisms and creates appropriate conditions for generating secondary biogenic gas. The gas is reserved in the karst caves and fractures in the L-1 limestone due to hydraulic migration and hydraulic plugging effects.