The invention relates to a solid electrolyte and a preparation method thereof. The solid electrolyte is formed by monoclinic phase lithium zirconate crystal admixed with M element, and the monoclinic phase lithium zirconate crystal admixed with the M element is that M ions are admixed to lithium niobate crystal and the composition of the chemical formula is Li6+yZr2-xMxO7, wherein X is equal to 0.01-0.4, and M is a positive divalent metal element or a positive trivalent metal element; when M is the positive divalent metal element positive, and Y is equal to 2x; and when the M is the positive trivalent metal element, and y is equal to x. The method comprises steps for preparing the monoclinic phase lithium zirconate crystal admixed with the M element by adopting a high-temperature solid-phase method and dry-pressure sintering, and has the advantages of high production efficiency and high yield and purity; the safety problem of a lithium-ion battery is solved thoroughly because of no use of any liquid organic solvent; and the prepared solid electrolyte has high ionic conductivity, excellent electrochemical stability, and a very wide application prospect.