The present invention a method for determining the gas saturation of a tight reservoir. The method comprises the steps of: determining the pore size distribution of the tight reservoir rock sample, and calculating the free water saturation; calculating the water-membrane water saturation; calculating the corner water saturation; calculating the gas saturation of the tight reservoir rock sample according to the following equation: Sg=100−Sw wherein Sw is the water saturation in %; Sw is the sum of the free water saturation, the water saturation and the corner water saturation; Sg is the gas saturation in %. The method for determining the gas saturation of a tight reservoir uses model calculations, which avoids errors in the determination results of the gas saturation caused by water volatilization, surface adsorption, and observation of water flow during experiments.