Unsteady reservoir, characterized by oblique oil-water contact or an upside-down relation between gas-oil-water in interconnected reservoirs, refers to a kind of hydrocarbon reservoir still in the process of hydrocarbon charge or adjustment in which a uniform oil-water contact does not come into being and there is a dynamic nonbalance between gas, oil, and water. This article takes Donghe sandstone in Hadson Oilfield in Tarim Basin as an example and makes the analyses on the origin of oblique oil-water contact, showing that the Donghe sandstone is undergoing hydrocarbon adjustment with obviously unsteady hydrocarbon concentration, and neotectonic movement is the major cause of the oblique oil-water contact. We put forward the concept of unsteady reservoir based on physical modeling, which has been applied in the hydrocarbon exploration and development of Hadson Oilfield. After successive exploration and development for nearly 3 years, the Hadson Oilfield has gained increasing reserves in place and become the first large-scale marine sandstone oilfield in China, with the proved reserves exceeding a hundred million tons. Key words: unsteady reservoir, oblique oil/gas/water contact, neotectonic movement, hydrocarbon adjustment, Hadson Oilfield, Tarim Basin