Abstract: The co-gasification technology of coal and biomass has attracted a widespread attention because of its remarkable performance in CO₂ emissions reduction, improving carbon reactivity and reducing tar content in gas production.The revelation of the role and rules of biomass and alkali and alkaline earth metals in biomass is the premise for the application and promotion of coal and biomass co-gasification technology.In this paper, the synergy in the co-gasification process is discussed, and the influence of alkali and alkaline earth metals on the synergy is reviewed.The release mechanism of alkali and alkaline earth metals during the thermal conversion of biomass is discussed.The influence of temperature, atmosphere, inorganic minerals etc.on the release of alkali and alkaline earth metals is summarized and the migration paths of potassium during the process of co-gasification of biomass and coal are described in detail.It focuses on the material transfer caused by the difference in the chemical composition and structure between coal and biomass and the resulting synergetic mechanism, including the hydrogen transfer of biomass to stabilize the structure of molecular fragments produced by coal pyrolysis, the heat of biomass thermal decomposition to promote the low-temperature thermal decomposition of coal, and the alkali metals, such as potassium, sodium and alkaline earth metals, calcium and magnesium etc.catalysis and deactivation mechanism during the gasification process of mixed chars.The influence of the interaction in the process of co-pyrolysis of raw materials and co-gasification of mixed char on the gasification reactivity of char is compared.The possible reactions between alkali and alkaline earth metals and sulfur species in the gasification process are introduced.Calcium has the strongest capture effect on sulfur, followed by potassium, but their availability depends on the amount of Si, Al and Cl elements in the material.Lastly, combined with the existing problems in the applied basic research, some suggestions are proposed for the investigation regarding the release, control and catalysis of alkali and alkaline earth metals in biomass, which would provide a theoretical basis for the alkali and alkaline earth metals to turn harm into advantage.