In this article, we present a theoretical study on the sub-bandgap refractive indexes and optical properties of Si-doped β-Ga₂O₃ thin films based on newly developed models. The measured sub-bandgap refractive indexes of β-Ga₂O₃ thin film are explained well with the new model, leading to the determination of an explicit analytical dispersion of refractive indexes for photon energy below an effective optical bandgap energy of 4.952 eV for the β-Ga₂O₃ thin film. Then, the oscillatory structures in long wavelength regions in experimental transmission spectra of Si-doped β-Ga₂O₃ thin films with different Si doping concentrations are quantitively interpreted utilizing the determined sub-bandgap refractive index dispersion. Meanwhile, effective optical bandgap values of Si-doped β-Ga₂O₃ thin films are further determined and are found to decrease with increasing the Si doping concentration as expectedly. In addition, the sub-bandgap absorption coefficients of Si-doped β-Ga₂O₃ thin film are calculated under the frame of the Franz–Keldysh mechanism due to the electric field effect of ionized Si impurities. The theoretical absorption coefficients agree with the available experimental data. These key parameters obtained in the present study may enrich the present understanding of the sub-bandgap refractive indexes and optical properties of impurity-doped β-Ga₂O₃ thin films.