Abstract: western Liaohe depression is one of the depressions rich in oil and gas resources in China. This paper mainly studies the geochemical characteristics of oil in its western depression, classifies its oil resource and analyses the origin of all kinds of oil.The result shows : In Liaohe deprssion basin, not only the depression can be an independent source system, but also each secondary depression in it can control the formation and accumulation of oil and gas. The secondary depression is an oil-formation.