The origin, source and evolution of the reef flat facies lithologic gas reservoir of Changxing Formation in Yuanba Gas Field were studied systematically by themethods of geologic and geochemical inversion. The result shows that the gas of the reel flat facies lithologic gas reservoir is mainly from the oil cracked gas of paleo-reservoir, and a small part of the gas is directly from the source rock of Upper Permian. During Late Triassic-Early Jurassic, crude oil migrated to reservoir along the interlayer fractures and joint fissures, and formed paleo-reservoir. During Late Jurassic-Late Cretaceous, the paleoreservoir cracked, gas migrated along fractures and pore reservoir and accumulated, and formed paleo-gas reservoir. Since Late Cretaceous, gas further migrated and formed the present gas reservoir. The gas reservoir experienced physical adjustment and chemical alteration of natural gas component. Small amplitude of uplift in the northwestern Yuanba Gas Field leads to the relatively shallow burial depth and the weak TSR response.