In this article，optical and electronic properties of CIS are studied from DFT calculations. For the exchange-correction energy，we employ GGA in the form of RPBE. The interactions between valence electrons and ionic core are represented by the ultrasoft pseudo potential. By analyzing the results，a mixed bonding state is found to exist in CIS，which is a strong ionic bond with a much weaker covalent bond. The interactions between Cu and Se are much stronger than that between In and Se. It was clarified that CuInSe2 has a direct band gap. We also calculate various optical properties of this material，such as reflection index，refraction index，adsorption spectrum and dielectric constant. For the adsorption spectrum，there are six peaks：3.1，7.6，10.0，16.1，19.0eV，21.0eV and the strongest adsorption peak is located in ultraviolet region.