The chemical composition, phase component and morphology characteristics of scale were researched by chemical analysis, XRD, SEM and TEM. The results show that the scale can be divided into iron minerals, silicon minerals, Ca-Ti-Mg minerals and aluminum hydroxide. Alumogoethite and micro-fine sodium-silicon slag fill in the gaps between the neat and bulky hematite crystals. Microgranular hydrated garnet and olivine fill in the gaps between the goethite crystals with a dense layered structure, accompanied by a small amount of galena and apatite precipitation particles. Perovskite and magnesium hydroxide have five periodic sedimentary cycles, forming an alternating sedimentary layer structure. Aluminum hydroxide forms thick scale and presents two kinds of typical structures, spherulitic and laminated. The research provides the theoretical basis for preventing and controlling scale in high-pressure digester group.