In this paper， high transmittance modulated NiO_x thin films were prepared by DC reactive magnetron sputtering method at room temperature by using indium tin oxide （ITO） glass as the substrate. The film was analyzed and characterized by X-ray diffractometer （XRD）， scanning electron microscope （SEM），UV-Vis spectrophotometer （UV 3600）， the effects of argon-to-oxygen ratio， air pressure and power on the electrochromic properties of the film during sputtering was studied. The results show that the prepared NiO_x film has obvious crystal grains and pores on the surface， and preferentially grows on the（200）crystal plane.With the increase of air pressure， power and oxygen partial pressure， the transmittance modulation first increases and then declines.When the time is 45 min， the sputtering pressure is 4.9 Pa， the argon-oxygen ratio is 113∶7，and the power is 215 W， the maximum can reach 58.3%. The coloring and fading response times are 9 s and 19 s，respectively， and the corresponding coloring and fading efficiency are 77.56 cm²/C and 40.39 cm²/C； the electrochromic devices with the structure of Glass/ITO/NiO_x/Li-electrolyte/WO₃/ITO/Glass are prepared with the optimal NiO_x film， and the light modulation amplitude of the device at 550 nm is 46.5%， the dyeing and fading times are 27 s and 45 s， the corresponding dyeing and fading rates are 1.5 %/s and 0.9 %/s， and the transmittance modulation remained above 40% after 500 cycles.