GeC/GaP doublelayer thin film is used as longwave infrared (8～11.5 μm waveband) antireflective and protective thin film system for ZnS substrates. Gallium phosphide thin film has been deposited by RF magnetron sputtering with high-purity Ar as the working gas and single crystal GaP disc as the target, and germanium carbon thin film has been deposited by reactive RF magnetron sputtering in high-purity Ar and CH4 mixture with single crystal Ge disc as the target as well. The thicknesses as well as the optical parameters of both GaP film and GeC film, such as refractivity and absorption coefficient are determined by least square fit of the films' IR transmission spectra, with the refractive indexes expressed in Cauchy formula and absorption coefficients in Urbach formula. The refractive index of GaP film is about 2.9 at 10 μm wavelength and is close to that of the bulk GaP; and that of the GeC film is about 1.78 at 10 μm wavelength. With these refractive indexes, GeC/GaP double-layer antireflective and protective thin film system is designed by computer and deposited on ZnS substrate. The GeC/GaP film system shows relatively large absorption when the GaP film is relatively thick, which makes the antireflective effect poor, whereas the antireflective effect is good when the GaP film is relatively thin.