A modified garnet-type solid electrolyte, includes: a garnet-type solid electrolyte; a modification layer, such that the modification layer is formed on at least one side of the garnet-type solid electrolyte, and possesses a three-dimensional crosslinking structure comprising at least one strongly acidic lithium salt and at least one weakly acidic lithium salt. A method of forming a modified garnet-type solid electrolyte, includes: exposing a garnet-type solid electrolyte in air to form a pre-passivation layer; mixing solutions of strong acid and weakly acidic salt to form a mixed solution; chemically treating at least one side of the garnet-type solid electrolyte with the mixed solution; and forming a modification layer on the at least one side of the garnet-type solid electrolyte.