Single-crystalline Zn1-xMgxO thin films with c-axis orientation have been deposited on Si（100） substrates by pulsed laser deposition. The photoluminescence characteristics of the films are studied by fluorescence spectrometer. The results show that the ultraviolet emission peak has a blue shift and the intensity weakens with the increasing content of Mg. At the same time，the intensity of defect emission increases with increasing content of Mg. Some local bound states were introduced by Mg-doping. For the samples grown in oxygen atmosphere，the results show that both the ultraviolet emission peak and green emission peak are enhanced，but the value of R reduced and the ultraviolet emission peak has a red shift. The study of green-emitting mechanism indicated that the green emission band mainly depends on zinc vacancy，substitutional O on the zinc site （OZn） and interstitial oxygen vacancies （Oi）. Green emission band is composed of many defect-peaks，and its movement as a whole mobile is due to the change of relative intensities of individual defect-peaks.