The invention provides a preparation method of an electrolyte used for a lithium manganate power battery at a high temperature, and relates to the technical field of lithium manganate power batteries. The components in the formula of the electrolyte comprise a non-aqueous organic solvent, a lithium salt, a high temperature additive, 0.1-5% by mass of a film forming additive, 0.1-10% by mass of a fire retardation additive, 0.1-10% by mass of an overcharge-preventing additive and 0.001-1% by mass of a stabilizer. The preparation method comprises: at a room temperature, weighing a film forming additive, a non-aqueous organic solvent, a high temperature additive, an overcharge-preventing additive, a fire retardation additive, a stabilizer and a lithium salt in a glove box filled with argon, pouring the raw materials into a beaker, placing on a magnetic stirrer, controlling the stirring rate until completely dissolving the lithium salt, and uniformly mixing various solvents so as to obtain the non-aqueous electrolyte, wherein the non-aqueous electrolyte has excellent high temperature cycle performance and excellent safety.