Abstract: Study of fluid inclusions is applied in the division of oil and gas migration filling period and the identification of reservoir forming time. With observation of microscopic characteristics, classification of types and description of features, the brine inclusions that associate with hydrocarbon are tested for homogenization temperature. Combining with other geological data, it is concluded that the homogenization temperature of fluid inclusions from the reservoir of Majiagou formation in Yanchang exploring area concentrates in the range of 85℃ to 115℃, with depth of 2500 m to 2800 m. In consideration of the burial history of the basin, the gas reservoir should be formed in Late Triassic to Early Jurassic era.