The method for manufacturing a solid electrolyte using an LLZ material for a lithium-ion battery comprises the steps of: providing a starting material in which lanthanum nitrate [La(NO3)3.6H2O] and zirconium nitrate [ZrO(NO3)2.6H2O] are mixed at a mole ratio of 3:2; forming an aqueous solution by dissolving the starting material; forming a precipitate by putting ammonia, which is a complex agent, and sodium hydroxide, which adjusts the pH of a reactor, into the aqueous solution, mixing the same, and then co-precipitating the mixture; forming a primary precursor powder by cleaning, drying and pulverizing the precipitate; forming a secondary precursor powder by mixing lithium powder [LiOH.H2O] with the primary precursor powder and ball-milling the mixture so as to solidify the lithium; and forming a solid electrolyte powder by heat-treating the secondary precursor powder.