We report the growth of single cubic phase Mg0.57Zn0.43O (MZO) alloy film through the method of metal organic chemical vapor deposition (MOCVD) and the relation between the quality and thermal stability of the alloy film after heat treatment. From X-ray measurement, we found that the quality of cubic MZO film was significantly influenced by the heating temperature. At 500-850℃, the crystallization and surface morphology of the alloy film were improved obviously as the temperature increased. Also, the blue shift of absorption cut-off edge, broadened band gap and maintained single cubic structure were found with increasing temperature. However, up to 950℃, mixed phases were formed in cubic MZO alloy film. For the photoresponse measurement of the MSM unit devices synthesized by the cubic MZO alloy film under 15 V bias, we found that the response peak of devices was around 260 nm, rejection ratio of UV/Vis was about 4 orders of magnitude, saturated responsibility was 3.8 mA/W and the value of dark current was about 5 pA.