The invention discloses a middle refractive index optical thin-film used evaporating material with the advantages of film layer compactness, firmness, good chemical stability and higher transmittance from the near ultraviolet to the near infrared, as well as a preparation method thereof. The evaporating material has the technical proposal that the middle refractive index optical thin-film used evaporating material is characterized in that the material is a fused material, and is a compound composed of the chemical formulas: LaAlO3 and LaAl11O18. The preparation method comprises the following steps: Al2O3 and La2O3 are taken as raw materials; the ratio of the raw materials is 24-78 percent (Wt) Al2O3; and a solid state reaction is carried out under high temperature and vacuum states. The use method of the middle refractive index optical thin-film used evaporating material comprises the following steps: charging and pre-melting are carried out in an oxygen-free copper crucible; the temperature for pre-melting is about 2200-2400 DEG C; the material is in the melting state after the pre-melting process; a substrate used for filming is a quartz substrate or other substrates; and a hard media film layer, a resin lens film, a dichroic prism or a broadband antireflection film is formed.