In this paper, transmission spectroscopy was used to measure the spectral curve of Diamond-like carbon film on Ge substrate. By using the measured spectral curve and simulated annealing algorithm, the objective optimization function was constructed, and the thickness, refractive index and extinction coefficient of the film are obtained by spectral inversion. Compared the optical parameters of Diamond-like carbon film on Ge substrate obtained by this method with the measured results of ellipsometer, the error of refractive index is less than 1%, the error of thickness is less than 2%. In addition, the optical parameters of the film were brought into the theoretical calculation model of transmittance. Compared with the actual measured curve, the error of the transmission spectrum curve of Diamond-like carbon film on Ge substrate is less than 2%. Once the transmission curve measured, the optical parameters of the film can be obtained by calculation.