Abstract: The measurement results of spectroscopic ellipsometry(SE) can not directly reflect the structure and optical properties of the tested samples,so the least square method for fitting the measurement data is needed,and the interpretation of the SE results is model dependent.Usually the fitting results are not only one.Thus,taking into account of the reasonableness of fitting model is important.The reasonableness of the model constant of the DLC film's optical constant is studied with the ellipsometry in this paper.We analyzed and fit the optical constant of the DLC film deposited by pulseion source,and investigated the reasonableness of the fitting model and reliability of the fitting result.