Abstract: The effects of the addition of CaO, temperature and residence time on the conversion and product yields of a highash peat in supercritical water was investigated. The experiments were carried out in an autoclave in the temperature range of 623K~773K at pressure up to 30MPa. At a Ca/C molar ratio of 0.46, almost no CO2 remained in gas phase. It was found that CaO facilitates the extraction of volatile matter from peat and the decomposition of volatile matter to small molecular compounds. Moreover, CaO catalyses the steam reforming reaction of hydrocarbons and the watershift reaction. The addition of KOH might favor the reforming reaction of oil product to hydrogen and carbon dioxide. Furthermore, the yield of oil reaches a maximum at the temperature approximately 723K. A threefold increase in the yield of oil product was observed under 36.5MPa compared with pyrolysis. However, the polar compounds decomposed with the prolongation of reaction time.