Hydrogenated amorphous silicon (aSi:H) films were deposited on quartz substrates by reactive facing target sputtering technique. The transmission and reflectance spectra of aSi:H thin films measured by UVVIS transmittance measurement were studied. The absorption coefficient α, therefore extinction coefficient κ, were determined by T/(1－R) method. The refractive index n and film average thickness d were determined from the upper and lower envelopes of the transmission spectra, and further modified by the basic equation for interference fringes. The dispersion curve of aSi:H film fitted by Cauchy dispersion relationship of refractive index was given. In order to illuminate the reliability of the determined parameters, the calculated transmission curve was compared to experimental one, the results show the two curves almost match together. Therefore the method is effective to determine aSi: H thin film thickness and optical constants.