The invention discloses a preparation method of a composite electrolyte sheet. The preparation method comprises the following steps: preparing a honeycomb alumina sheet, namely, mixing alumina powder, tert-butyl alcohol, an adhesive, a dispersant and deionized water, carrying out ball milling, injecting a mixture into a die, carrying out solidification, vacuum freeze-drying, sintering and cutting to obtain the honeycomb alumina sheet; then preparing LLZTO slurry, namely, weighing lithium carbonate, lanthanum oxide, zirconia and tantalum oxide, carrying out ball milling with ethanol, drying to obtain a premix, carrying out heat treatment on the premix to obtain preheated powder, mixing the preheated powder, ethyl glycol, the adhesive and triolein, and carrying out ball milling to obtain the LLZTO slurry; and finally, preparing the composite electrolyte sheet with a honeycomb alumina supported LLZTO membrane, namely, coating the LLZTO slurry on one surface of the honeycomb alumina sheet, naturally drying, putting on a crucible lid, and sintering to obtain the composite electrolyte sheet. According to the preparation method disclosed by the invention, the prepared composite electrolyte is excellent in performance and has wide application prospect in the field of lithium batteries.