Abstract: Electrochromic MoO₃ thin films were prepared on the PET/ITO substrates by the sol-gel method.The structure and thermal stability of the films were analyzed by FT-IR and TG-DTA.The electrochromic properties were measured in a three-electrode cell with a 1 mol/L LiClO₄ /propylene carbonate electrolyte by spectrophotometer and electrochemical work station.The results show that the films annealed at 120℃ are composed of amorphous MoO₃,H₂O₂ and H₂O.The transmittance varies between 75% and 35%,and the colouration efficiency at 550nm is 44.8cm²/C.The electrochromic properties are quite stable after 1000 colour/bleach cycles.With the increase of the applied voltage,the colouring speed increases and the contrast intensifies.The colouring speed is controlled by the diffusion of lithium atoms,while the bleaching speed by the transfer of lithium ions.