NOVELTY - A lanthanum molybdate-based solid electrolyte thin-film material includes lanthanum molybdate-based material. The substrate is coated with solid electrolyte polycrystalline thin-film comprising lanthanum molybdate-based material. The thickness of the solid electrolyte polycrystalline thin-film is 50 nm-10 mu m. The solid electrolyte polycrystalline thin-film comprises lanthanum molybdate-based particles having grain diameter of 10-90 nm.    USE - Lanthanum molybdate-based solid electrolyte thin-film material for mesothermal fuel cell, oxygen sensor as solid electrolyte and electrode material.    DETAILED DESCRIPTION - A lanthanum molybdate-based solid electrolyte thin-film material comprises substrate and lanthanum molybdate-based of formula (La2-xAx)(Mo2-yBy)O9-delta. The substrate is coated with solid electrolyte polycrystalline thin-film comprising lanthanum molybdate-based. The thickness of the solid electrolyte polycrystalline thin-film is 50 nm-10 mu m. The solid electrolyte polycrystalline thin-film comprises lanthanum molybdate-based particles having grain diameter of 10-90 nm.    A=La site dopant, preferably K, Ba, Gd, Nd, Bi, Ca, Fe, Mn, W, Re, Cr, V or vacant site;    x=0.03-0.3;    B=Mo site dopant; and    y=0.03-0.5.    An INDEPENDENT CLAIM is included for a method for preparing lanthanum molybdate-based solid electrolyte thin-film material comprising:    (A) weighing lanthanum oxide, oxide of lanthanum site dopant or nitrate or acetate, ammonium molybdate and oxide of molybdenum site dopant or nitrate or acetate according to ratio of (La2-xAx)(Mo2-yBy)O9-delta, dissolving lanthanum oxide in the nitric acid, and preparing other materials into corresponding aqueous solutions, dropping lanthanum nitrate solution to stirred aqueous solution, adding lanthanum site dopant solution and molybdenum site dopant solution to obtain mixed solution, adding citric acid to the mixed solution, stirring, where molar ratio of total metal ion in citric acid to the mixed solution is 1:1-3, placing at 60-100 degrees C stirring for 2-10 hours to form clear sol, and filtering the clear sol to obtain sol;    (B) mixing the sol with soluble high molecular polymer, adding water to regulate to make total concentration of metal ion at 0.01-0.2 mol/L, and making the high molecular polymer to 20-80 g/L, then adding citric acid to regulate pH value to 0.5-4, and stirring to obtain coating colloid;    (C) coating the substrate with coating colloid, rising temperature of substrate coated with coating colloid to 600-650 degrees C and insulating for 10-50 hours at 550-600 degrees C, repeating the step for more than once to obtain lanthanum molybdate-based solid electrolyte thin-film material.