A method for preparing an inorganic solid electrolyte composite slurry includes: mixing an inorganic solid electrolyte powder with a first solvent and wet grinding an obtained mixture to form a preparatory slurry A; mixing a binder with a second solvent to form a preparatory slurry B; mixing the preparatory slurry A with the preparatory slurry B to obtain the inorganic solid electrolyte composite slurry. The inorganic solid electrolyte composite slurry prepared by the present disclosure effectively solves the problem that it is difficult to reduce the inorganic solid electrolyte powder particle size in the preparation process, or it is difficult to fully dry the inorganic solid electrolyte powder after sand milling. The present disclosure further provides an inorganic solid electrolyte composite slurry prepared by the method, an application of the inorganic solid electrolyte composite slurry, and a lithium-ion battery having the inorganic solid electrolyte composite slurry.