Optical transition properties of trivalent rare earth ions in transparent luminescent materials can be investigated via Judd-Ofelt analysis. The traditional Judd-Ofelt analysis can not be applied to nontransparent powders since its absorption spectra are unable to be measured. Therefore, in this study, a procedure for calculating the Judd-Ofelt parameters was proposed by using defuse-reflection spectrum. Firstly, the defuse-reflection spectrum was transformed into relative absorption spectrum based on Kubelka-Munk function, and then the relative Judd- Ofelt parameters were confirmed from the relative absorption spectrum, at last the actual Judd-Ofelt parameters were obtained by using fluorescence decay data for calibrating the relative values. The method proposed for Judd-Ofelt parameters’ calculation was applied to Lu₂O₃:Er³⁺ phosphor prepared by solid-state reaction, and the method was confirmed. The obtained Judd-Ofelt parameters were examined by comparing the radiative transition rates of ⁴I_13/2 derived from the absorption cross section with which obtained from usual route by using the Judd-Ofelt parameters. The present study demonstrated that the proposed Judd-Ofelt analysis route is reliable and practical.