Due to the decrease in light crude oil and the ever-increasing demand for the upgrading of heavy oils, the development of new heavy oil processing technologies has been attracting wide attention. Using supercritical fluids (SCFs) as the reaction medium has great advantages for upgrading heavy oils. The upgrading processes which employ supercritical water (SCW) without catalysts and the effects of different operating conditions are summarized in this work. The temperature and the density of SCW are of great significance to upgrading processes. The upgrading process through partial oxidation and the water gas shift reaction is further discussed. The processes employing other SCFs with different kinds of catalysts are also summarised. Additionally, the role of the SCFs during the upgrading is discussed. According to the development of the upgrading of heavy oils with SCFs, some strategies are proposed to improve the quality of the products and to reduce the coke yield.