The electrochromic performance of sputter-prepared WO₃ thin films with an Al₂(SO₄)₃  aqueous electrolyte was investigated. The concentration of Al₂(SO₄)₃  aqueous solution was varied from 0.025 to 0.005 mol/L. As the concentration decreased, the op-tical modulation increased slightly, the response time was prolonged, and the coloration efficiency (CE) decreased slightly. However, the cycle life was significantly increased by reducing the Al³⁺ concentration. The retentions of the charge density and optical modu-lation at a concentration of 0.005 mol/L were over three times those at 0.025 mol/L. The optimal electrochromic performance of the WO₃ thin films was observed in a 0.005 mol/L Al₂(SO₄)₃  aqueous solution, with high optical modulation of 76.3% and CE of 80.14 cm²/C; reasonable response time of 10.6 and 13.6 s for coloring and bleaching, respectively; and long-term stability (102.7% retention of optical modulation after 200 cycles). Electrochemical impedance spectroscopy (EIS) suggested that the charge-transfer rate exceeding the ion mass transfer rate was conducive to the stability of the electrochromic cycle of the WO₃ thin films.