The effects of sputtering Ar pressure on optical constants (n,k) of Ge2Sb2Te5 thin films in the wavelength range from 300 nm to 900 nm were studied. The results show that the refractive index (n) first increases and then decreases with increasing Ar pressure, whereas the extinction coefficient (k) changes with Ar pressure in a contrary way to that of n. The extent of the influence of Ar pressure on n and k also changes with wavelength. It is greater in the long wavelength region than in the short wavelength region. The mechanism on which the optical constants of the Ge2Sb2Te5 thin films are affected by the Ar pressure is discussed based on the variation of the atomic density and microstructure of the films.