A novel errors treatment technique to optical-parameters characterization of thin films is presented based on the effect analysis of systematic errors which are hard to eliminate in ellipsometric measurement data. In order to minimize the characterization deviations of thin film optical parameters from the real values caused by ellipsometric measurement systematic errors, it is advised to select ellipsometric measurement data used in optical characterization from spectral bands characterized with opposite signs or single zero of first-order ellipsometric angles′ partial derivatives with respect to layer thickness and refractive index for most measurement incident angles, and to exclude spectral bands characterized with the same signs or both zeros of ellipsometric angles′ first-order partial derivatives for all measurement incident angles. The essence of this technique is to minimize the errors transfer effect of measurement data on thin-film optical-parameters characterization through spectral band selection of ellipsometric measurement by first-order partial derivatives analysis. Through numerical simulations, its applicability and selection skills of the range of measurement incident angles are compared and studied. The reliability of this technique is supported by resumable numerical experimental results and reasonable theoretical explanations. This novel errors-treatment technique provides a certain reference to in-situ characterization and thickness monitoring of thin films.