It is necessary to select appropriate thickness for the fabrication of Bi₂O₃ thin films with reasonable electrochromic properties. Bi₂O₃ thin films with an approximate range of 20∼300 nm in thickness were prepared by magnetron sputtering. The electrochromic properties of the films were analyzed by using ultraviolet-visible (UV-Vis) spectrophotometry and an electrochemical workstation. The surface morphology and phase structure of the thin films were investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD). After comprehensive screening to assess their performance in terms of the color change contrast (∆Tλ=550 nm), electrochromic efficiency (η), and performance retention (R∆T , R_η ), the values of ∆T and η of Bi₂O₃ thin films with a thickness between 60 nm and 120 nm were found to reach 25% and 10 cm²/C, respectively. Furthermore, the retention of the electrochromic properties (R∆T = 20%, R_η = 44.6%) is high. This may be related to the fact that the main phase of the film in this thickness range is δ, which is characterized by high ionic conductivity.