Abstract: The physical and geochemical properties of viscous oil in the northwest margin ofJunggarBasin were studied in the paper.By correlating it with viscous oils home and abroad,it isfound that the viscous oil in this region is characterized by geochromatographic effects resulted from the long distance migration of hydrocarbons,Additionally,the alteration ofviscous oil would be divided into biodegradation,oxygenation,water washing and diffu-sion respectively.