The internal factors leading to the degradation of proton based all solid state electrochromic device were discussed. Two factors were found for the degradation of EC device: in the bleaching process, water contained in WO 3 thin film would cause OH - ions to accumulate in the film and lead to an alkaline environment, Wo 3 would be solvable in it and form tungstate; Electrolysis leading to the evolution of gas of hydrogen and oxygen would happen in the EC device under high voltage, and finally the film's peeling off. Through the improvement of the device's structure and the deposition technique, an good properties and long-cycle-life all-solid-state electrochromic device with ΔO.D. of 0.5 and C/B cycles beyond 106 was gained.