Abstract: Based on the gas data during geological exploration and mining period,combined with experimental results of adsorption constant,porosity and industrial properties,the paper analyzed the factors affecting gas occurrence from coal properties to coal-forming environment and geological structures by using methods of gas-geology;and it pointed out that,though the coal-forming environment is conducive to gas occurrence,the geological structure is the controlling factors causing low gas occurrence in No.3 coal seam.Under the special condition,gas occurrence should be analyzed with geological background concretely.