Thin films of WO3:H (similar to 450 nm) were prepared on indium fin oxide (ITO) coated glass by reactive direct current magnetron sputtering at room temperature. The visible transmittance of the original state, and the X-ray photoelectron spectroscopy analysis of WO3:H films prove that H+ ions have been introduced into the structure of WO3. Based on the optimized parameters of WO3:H thin film, the Glass/ITO/WO3:H/Ta2O5/NiOx/ITO electrochromic devices (ECDs) are prepared monolithically by magnetron sputtering. Different applied voltages of 1.0, 2.0 and 3.0 V were applied to the ECD for investigating its influence on the ECD's optical contrast and cycle life. Though the ECD can get a higher contrast about 42% at 550 nm under 3.0 V voltage, however, it will be destructed quickly. When the operating voltage is 2.0 V, the ECD has a stable cycle performance with the optical contrast of > 30%.