NOVELTY - A rare-earth fluoride is prepared in air by selecting rare-earth oxide as raw material, dissolving prepared rare-earth oxide by acid and deionized water to form rare-earth salt solution, adjusting pH value of rare-earth salt solution to be 6-7, dissolving precipitant into deionized water to obtain precipitant solution, adding excessive precipitant solution into rare-earth salt solution and stirring, putting aside to obtain rare-earth fluoride precipitate, and washing precipitate until pH value of cleaning solution is 6-7.    USE - Preparation of rare-earth fluoride for laser process, optical fiber amplifier, anti-fake material, tricolor display material, light conversion film of solar battery, and fluorescent markers for biological detection and analysis.    ADVANTAGE - The method has simple operation and good repeatability, and obtains rare-earth fluoride which has uniform distribution, good dispersion and high stability.    DETAILED DESCRIPTION - A rare-earth fluoride is prepared in air by:    (A) selecting rare-earth oxide as raw material, dissolving prepared rare-earth oxide by acid and deionized water to form rare-earth salt solution, and adjusting pH value of rare-earth salt solution to be 6-7;    (B) dissolving precipitant into deionized water to obtain precipitant solution, adding excessive precipitant solution into rare-earth salt solution and stirring, putting aside to obtain rare-earth fluoride precipitate, and washing precipitate until pH value of cleaning solution is 6-7;    (C) adding distilled water into rare-earth fluoride precipitate, stirring to form turbid liquid, adding fluoride of metallic element into turbid liquid, obtaining mixed solution, heating mixed solution at 80-120 degrees C in air, stirring for 6-360 hours, and obtaining rare-earth fluoride;    (D) annealing rare-earth fluoride for 1-2 hours in tabular furnace at 200-1000oC and under condition of hydrogen, nitrogen or vacuum; and    (E) obtaining rare-earth fluoride with particle size of 10-1500 nm in which stoichiometric equation of rare-earth fluoride is ( Sigma M)x( Sigma Ln)yFz.    x,y= not less than 0-2;    z=4-8;    Sigma M=metallic element; and    Sigma Ln=rare-earth element.    DESCRIPTION OF DRAWING(S) - The drawing shows a graph regarding preparation of rare-earth fluoride. (Drawing includes non-English language text).