The optical properties: dielectric function, refractive coefficients, absorption and reflection spectra, etc. for NH₄ClO₄ crystal were calculated by the first principle method based on the density functional theory (DFT). The relationship between peaks in the dielectric function pictures and the inter-bands transitions in the band structures was analyzed. Results show that the static dielectric function ε₁(0) is 1.10, and the static reflectivity n₀ is 1.05, and the maximum absorption coefficient is 1.91×10⁵ cm^-1 corresponding to peak energy of 10.88 eV. The calculation results are in good agreements with experimental ones.