Disclosed is an electrolyte for an electrochemical device. The electrolyte includes a composite of a plastic crystal matrix electrolyte doped with an ionic salt and a crosslinked polymer structure. The electrolyte has high ionic conductivity comparable to that of a liquid electrolyte due to the use of the plastic crystal, and high mechanical strength comparable to that of a solid electrolyte due to the introduction of the crosslinked polymer structure. Further disclosed is a method for preparing the electrolyte. The method does not essentially require the use of a solvent. Therefore, the electrolyte can be prepared in a simple manner by the method. The electrolyte is suitable for use in a cable-type battery whose shape is easy to change due to its high ionic conductivity and high mechanical strength.