Abstract: Isolated island gas pool is a kind of shallow gas pools with low pressure, and the porosity of its capping mudstone bed all exceeds 6%. Using mercury injection-gas absorption method, several characteristic parameters have been obtained: the distributive patterns of abrupt pressure, mean radius and micropore radius,and the saturation under the capping bed, the gas column height, and shielding factor. Combined with geological characteristics of gas field in Jiyang depression, the capping bed of isolated island shallow gas pools are quantitatively evaluated. The effects of capping bed are discussed, proposing that there must be certain regular changes in the enrichment coeficient of reserves, the contents of natural gas components and the pressure coeficient of gas reservoir due to the existence of capping bed. With the application of characteristic parameters for capping bed, the formative preserving mechanisms for isolated island shallow gas pools are analysed, suggesting good correlations of the initial formation pressure, gas bearing saturation and elastic yields with abrupt pressure, shielding factor, gas column height of the capping bed.