The invention relates to a determining method of the gas saturation in a gas reservoir, and belongs to the technical field of unconventional exploration and development of gas reservoirs. The influences of geological factors such as the gas reservoir type, the reservoir characteristics and the fluid distribution under the actual gas reservoir condition are considered, the gas-drive-water process in a gas reservoir is simulated in a lab, analytic samples are screened through the collection of rock samples with complex pore structures in the very low porosity and permeability gas reservoir, the analytic data of the analytic samples are determined, the relation between the water saturation Sw and the average capillary pressure of the analytic samples and the relation between the water saturation Sw and the average capillary pressure of the gas reservoir are established in the lab, and according to the relation between the height H of a gas column above a free water surface of the very low porosity and permeability gas reservoir and the water saturation Sw, the gas saturation Sg of the very low porosity and permeability gas reservoir is determined economically and accurately.