Developing growth techniques to suppress phase separation is a key point to realize bandgap modulation of MgZnO from 3.3 to 7.8 eV. A graded quasi-homo buffer layer, composed of a MgO layer and a continuously Zn-increased MgZnO layer, was used to grow pure cubic Mg Zn O thin films on c-plane sapphire by MOCVD method. The Zn content in rocksalt Mg Zn O is expanded to ~0.7 and the optical bandgap of the films was decreased as low as 4.45 eV, corresponding to the long wavelength edge of solar-blind band. Parallel experiments suggested that the buffer layer played an important role as a structural template for the following growth of Mg Zn O epilayers and a substrate temperature range from 350℃ to 400℃ was benefit to increase solution of Zn in the RS-MgZnO films. A photoconductive UV detector based on a Mg Zn O film exhibits responsivity peak at 270 nm and a cutoff wavelength at 295 nm.