Abstract: Aluminum oxide thin films were deposited through the medium-frequency reactive magnetron sputtering process. An effective and simple way was therefore developed to analyze the optical properties of low-absorbability films and other relevant physical properties by use of transmittance spectroscopy for Al2O3 film samples. The dependences of refractive index and extinction coefficient on wavelength were calculated in this way. The interrelation between incident light wavelength, refractivity, absorptance and film thickness of Al2O3 films is discussed, thus giving the relationship between the refractivity, absorptance and incident light wavelength in the range of 400-1000 nm, as well as the formula of Al2O3 film thickness.