Antimony-based bismuth-doped films with different thicknesses are deposited by magnetron sputtering method. The structure of samples with different thickness is studied by X-ray diffraction (XRD) and transmission electron microscope (TEM). The optical constants and optical band gap of the samples in range of 950～2200 nm are measured by spectroscopic ellipsometry. The influence of thickness on optical constants and optical band gap is investigated. The results show that the structure of the samples transforms from amorphous state into crystalline state when the thickness increases from 7 nm to 100 nm. In wavelength range of 950～2200 nm, the refractive index, extinction coefficient and optical band gap of the samples with different thickness are in range of 4.6～8.9, 0.6～5.8 and 0.32～0.16 eV, respectively. The refractive index and optical band gap decrease and the extinction coefficient increases with the increase of thickness. At thickness of 50 nm, the optical constants have a critical value, which results from the change of microstructure when thickness is larger or smaller than the critical value.