The optical properties of PbZr_xTi_1-xO₃(x=0.52) amorphous thin films on vitreous silica substrates by RF magnetron sputtering were investigated by UV～VIS～NIR transmittance measurement in the wavelength range of 200～1100nm. Meanwhile, based on the structure of thin films on the transparent substrate and the transmittance relations of multilayer structure, the fitting method with six fitting parameters was developed to calculate the optical constants. The optical constants of thin films in the wide wavelength range and the film thickness were obtained simultaneously. The maximum values of the refractive index and extinction coefficient are 2.68 and 0.562, respectively. The film thickness by the fitting is 318.1nm. According to Tauc′s law, the direct band gap of the PZT amorphous thin film is found to be 3.75eV. Finally, the dispersion relation of the refractive index of the thin films is successfully interpreted by considering a single-oscillator model.