During the supercritical water gasification process, the dissolution characteristics of inorganic elements existing in biomass affected not only the gasification transformation of the organic matter but the safety of the system. In this study, the dissolution characteristics of inorganic elements existing in biomass, including corn cob, rice husk, soybean straw, and wheat straw, were carried out at a high-pressure autoclave. The amount of the major inorganic elements that dissolved out was determined by measuring the liquid product. Compared with the conventional water-leaching method, it was found that the dissolved amounts of K, Ca, p, and Mg were reduced, but Na and Si increased. Besides, some operational parameters were respectively considered. The results revealed that the low reaction temperature, low reaction pressure, short residence time, and low feedstock concentration promoted the dissolution of K and Mg, but hindered that of Na. Further, the results also showed that dissolution characteristics of Ca and P related to the biomass species. However, the dissolution characteristics of Si after the supercritical water gasification process were almost at the same level.