Ytterbium-doped yttrium oxysulfide (Yb:Y2O2S) has been synthesized by solid-state reaction with sulfide flux. Diffuse reflection and emission spectra have been measured in order to determine the crystal field splitting of Yb3+ ion in the YOS lattice. According to the crystal-field levels probed in the spectra, the crystal field splitting of 2G7/2 manifold of Yb3+ ion in YOS is 709 cm-1, which is large enough for the quasi-three-level laser operation of Yb3+ ion. Emission peak position, width, full-width at half maximum (FWHM), and normalized intensity of Yb:YOS are fitted from its emission spectrum. For comparison, relevant data of 5.4 at.-% Yb:YAG is also provided.