The potential of diosgenin solid waste (DSW) to be a proper feedstock for hydrogen production from supercritical water gasification was assessed through thermodynamic analysis and experimental study. The thermodynamic analysis of DSW gasification in SCW was performed by Aspen Plus software based on the principle of minimum Gibbs free energy. The effects of temperature (500–650 °C), flow ratio of feedstock slurry to preheated water on the gasification were studied. K2CO3 and black liquor were used to catalyze the gasification of DSW. The morphological structures of DSW and residue char were characterized by SEM. The results showed that DSW was almost completely gasified at 650 °C without catalyst and the carbon gasification efficiency reached up to 98.55%. K2CO3 could significantly promote the gasification reactivity of DSW at a lower temperature. H2 yield was remarkably improved by adding black liquor. The SEM analysis indicated that parts of the organic matters reacted to form gases and liquid products, and K2CO3 was found to migrate into the residue char during the reactions.