The effects of sputtering parameters on optical constant of NiOx thin films prepared by reactive magnetron sputtering were studied by spectroscopic ellipsometry, XRD and XPS methods. The optical constant of the deposited films became smaller with O2/Ar flow ratios increasing. After annealing, the refractive index shows an increase, especially at a high O2/Ar flow ratio, while the extinction coefficient reduced by about 50%. The refractive index is greater for a higher sputtering power while smaller for a higher work pressure. It is related to the existence of interstitial oxygen and vacancy of Ni, the decomposition of NiOx and compactness degree of NiOx films, respectively.