The invention provides a photothermal conversion nanometer material. The chemical formula of the photothermal conversion nanometer material is AR1-x-yF4, wherein Ybx and Ery are doped in the AR1-x-yF4, A is at least one of Li, Na or K, R is at least one of Y, Gd, Lu, or Nd, x is less than or equal to 0.6 and greater than or equal to 0.01, and y is less than or equal to 0.1 and equal to or greater than 0.01; the particle size of the photothermal conversion nanometer material particles is within the wavelength range of 5-40nm. The photothermal conversion nanometer material can emit visible light with the range of 520-660nm under the excitation effect of the near infrared laser with the wavelength range of 750-1100nm, while a temperature rise of about 5-300 DEG C can be achieved, and since the ratio of the emission band with the wavelength of 525nm and emission band with the wavelength of 545nm in the emitted light and the temperature meet a good exponential relationship, the temperature detection function can be realized. The photothermal conversion nanometer material has the significant characteristics that the material can absorb near-infrared laser with the wavelength range of 750-1100nm and the photothermal conversion, upconversion luminescence and temperature detection functions are achieved simultaneously. Another object of the invention is to provide a preparation method and application method of the photothermal conversion nanometer material, and the preparation method is simple, easy to operate, free of pollution and low in cost.