Abstract: The catalytic gasification/co-gasification technology of carbon containing substances is an important technological approach to achieve clean and efficient utilization of carbon based resources, and is also a key research and development direction for new gasification technologies. Biomass ash is a derivative of biomass gasification/combustion processes, with a wide range of sources and high alkali metal/alkaline earth metal content. It is a high-quality alternative to metal catalysts in catalytic gasification processes and a naturally derived catalyst in co- gasification processes. It is crucial to systematically understand the catalytic behavior and mechanism of biomass ash in catalytic gasification/co-gasification processes for the long-term safe, stable, and efficient operation of actual industrial plants. This article provides an overview of the chemical composition of biomass ash, summarizes the effects of raw material types, addition ratios, gasification temperatures, and other factors on the catalytic behavior of biomass ash in the catalytic gasification/co-gasification process of carbonaceous materials, and summarizes multiple biomass ash catalytic mechanisms based on oxygen transfer, active intermediates, and semi coke structure evolution. Finally, the future research directions of biomass ash catalysis/co-gasification technology were discussed.