[en] Highlights: • Accurate ε of MoS₂ is obtained by both point-by-point method and Lorentz fitting. • Transition energies are extracted from Lorentz fitting and explained physically. • The evolution of optical properties with film thickness has been revealed. • Film thickness can be quantitatively controlled via sputtering time modulation. - Abstract: As a promising candidate for applications in future electronic and optoelectronic devices, MoS₂ has been a research focus in recent years. Therefore, investigating its optical properties is of practical significance. Here we synthesized different MoS₂ thin films with quantitatively controlled thickness and sizable thickness variation, which is vital to find out the thickness-dependent regularity. Afterwards, several characterization methods, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman spectroscopy, photoluminescence (PL), optical absorption spectra, and spectroscopic ellipsometry (SE), were systematically performed to character the optical properties of as-grown samples. Accurate dielectric constants of MoS₂ are obtained by fitting SE data using point-by-point method, and precise energies of interband transitions are directly extracted from the Lorentz dispersion model. We assign these energies to different interband electronic transitions between the valence bands and conduction bands in the Brillouin zone. In addition, the intrinsic physical mechanisms existing in observed phenomena are discussed in details. Results derived from this work are reliable and provide a better understanding of MoS₂, which can be expected to help people fully employ its potential for wider applications.