MgxZn1-xO has been an object of intense study as a novel photo-electronic function material in recent years. Mg0.25Zn0.75O film was prepared on quartz glass substrate by sol-gel method. Structure and optical properties of Mg0.25Zn0.75O film were studied both in theory and by experiment. The result indicates that Mg0.25Zn0.75O film has hexagonal wurtzite structure. The film is homogeneous and the average grain size is about 20nm. Absorption spectrum indicates that the absorption edge starts at 360nm and the corresponding forbidden band width is 3.83eV. The luminescence spectrum of the sample is composed of three peaks at 384.9， 444.8 and 533.6nm respectively and the peak of excitation spectrum is at 378nm. The crystal lattice of Mg0.25Zn0.75O film is increased， the forbidden band width is broadened and the ultraviolet and blue-green luminescence peaks red-shift about 59， 14 and 12.6nm， respectively， due to the interstitial defects introduced by the surplus Mg ions.