The present disclosure relates to an ionic liquid-based quasi-solid-state electrolyte in a lithium battery and a preparation method thereof. The quasi-solid-state electrolyte is of a porous network structure, which is obtained by a condensation reaction of a lithium salt, ionic liquid, a silane coupling agent and a catalyst, and has a high ionic conductivity. The quasi-solid-state electrolyte can stabilize a stripping/deposition process of lithium metal and inhibit growth of lithium dendrites, and shows a low overpotential and long-term cycle stability in a constant current polarization process. The interface impedance of a lithium metal sheet and the quasi-solid-state electrolyte is low, and is hardly increased with the age of the battery.