Based on the application demands of metallic oxide thin films in the mid-infrared, the optical constants dispersion characteristics of four kinds of metallic oxide films of TiO2, HfO2, Ta2O5 and Y2O3 in the mid-infrared (2.5~5 μm) region in moisture state are studied, the four kinds of metallic oxide films are prepared on the super polishing silicon substrates using electron beam evaporation deposition technology, and based on the Lorentz oscillator dielectric constant dispersion model, the optical constants of the four kinds of oxide thin films are calculated using the transmittance spectra inversion method. The results show that all the four kinds of oxide films have a small amount of water molecules and hydroxyl. In increasing order of water content of these films are TiO2, HfO2, Ta2O5 and Y2O3, and far away from the location of the water absorption, in increasing order of extinction coefficient of these films are TiO2, HfO2, Ta2O5 and Y2O3. In order to reduce the effects of water absorption, TiO2 and HfO2 are the comparative ideal metallic oxide materials in the mid-infrared region under the condition of the electron beam evaporation deposition process.