The composition and content of glassy phase in coal gasification slag ( CGS) change due to the difference in coal speciesand gasification process. I order to study the effect of glassy phase in CGS on mechanical properties and micromorphology of alkali-excit⁃ed cementitious material, a CGS of shell gasifier was used as raw material, and the content and composition of glassy phase in CGSwas changed by crystallization process at high temperature. The results show that the compressive strength of CGS based cementitious mate⁃rial can reach 30 MPa under the optimal alkali excitation condition. High temperature induced crystallization causes the formation of anor⁃thite in CGS, which not only decreases the content of glassy phase, but also increase the Si/ Al of glassy phase. In alkaline environment,anorthite does not take part in dissolution-polymerizationreaction, and less glassy phase in slag leads to decreasing dissolution of Si andAl, which is detrimental to mechanical properties. Besides, the mechanical property of cementitious material is linearly related to contentof glassy phase in CGS. Si/ Al of glassy phase decides the dissolution ratio of Si and Al under alkaline condition, which is useful for choiceof alkali activator to increasethe formation of gel in cementitious material. The micromorphology analysis shows that the surface of coal gasi⁃fication slag with high glassy phase is smooth, and more gel is generated in alkaline condition. During curing process, the microstructureof cementitious material becomes dense, which lead to increase of mechanical strength.