A method for preparing a sulfide-based solid electrolyte which is stable upon exposure to the air is provided. Specifically, a stabilization layer is formed on the surface of a sulfide-based solid electrolyte particle through treatment with a reactive gas.The sulfide-based solid electrolyte with superior air stability can be obtained because oxidation or reduction reactions with water, etc. in the air occur on the stabilization layer rather than on the sulfide-based solid electrolyte particle.