The invention relates to a preparation method for a garnet type solid electrolyte material. The preparation method comprises the following steps: mixing garnet type electrolyte powder, a binder and solvent so as to obtain electrolyte slurry; coating a PET film with the obtained electrolyte slurry and carrying out drying at a set drying temperature to partially evaporate the solvent so as to obtain an electrolyte film; peeling the electrolyte film from the PET film and cutting the electrolyte film according to a certain specification; and discharging the binder and the residual solvent in the cut electrolyte film so as to obtain a compact garnet type lithium-ion conductor electrolyte sheet. The preparation method provided by the invention can effectively overcome the problem that an electrolyte sheet prepared by using a conventional all-solid electrolyte sheet preparation method has large thickness and poses influence to the electrical performance of an all-solid-state battery.