The invention discloses a microemulsion method for preparing nanometer cathode Ln2NiO4 of a solid oxide fuel cell, which relates to a preparation method of cathode material Ln2NiO4 of a solid oxide fuel cell, and aims to solve the problems of poor chemical stability and high polarization resistance of a cathode material of an existing solid oxide fuel cell. The preparation method comprises the following steps of: (1) mixing and stirring an oil phase, a surface active agent and a cosurfactant to obtain a solution, and dividing the solution into two equal parts; (2) dissolving soluble lanthanide nitrate and nickel nitrate in water, and adding an obtained solution into a solution obtained in the step (1); (3) dissolving alkali into water, and adding an obtained alkali solution into the other solution obtained in the step (1), thereby obtaining microemulsion; and (4) mixing the two microemulsion, centrifuging and washing the microemulsion to obtain a solid-phase matter, and sintering the solid-phase matter at a high temperature to obtain the nanometer cathode Ln2NiO4. The polarization resistance of the nanometer cathode Ln2NiO4 is 0.5ohm.cm<2>, and the chemical stability of the nanometer cathode Ln2NiO4 is good. The microemulsion method provided by the invention is mainly applied to cathodes of solid oxide fuel cells.