Al （Al2O3，2wt%） doped ZnO thin films （ZAO） with tunable band gap，alloyed with different Mg contents （1wt%，3wt% and 5wt%），were prepared by radio frequency magnetron sputtering on glass substrates and annealed at 500℃ in air. The optical and electrical properties of the films were measured by X-ray diffractometer，four-point probe method，Hall-effect configuration，and UV-VIS spectrophotometer. The results show that the Mg co-doped ZAO thin films have a wurtzite structure with （002） preferred orientation. Compared with ZAO film，the optical transmission of Mg co-doped ZAO thin films in the ultraviolet region increases and the average optical transmissions in the visible region is greater than 85%，except the 5wt% Mg doped films. The band gaps of ZAO and Mg co-doped ZAO thin films can be modulated from 344 eV to 351 eV by increasing the Mg content from 1wt% to 5wt%. For 1wt%，3wt% and 5wt% Mg doping，the Mg co-doped ZAO thin films have the resistivity of 12×10-3，37×10-3 and 85×10-3 Ω·cm，respectively. Stokes shifts of 20，14 and 50 meV were observed for doping of 1wt%，3wt%，and 5wt% Mg contents. We find that a large Mg doping content will degrade the crystal quality of Mg co-doped ZAO thin films，decrease the optical transmissions in the visible region and decrease the carrier mobility in the films. Therefore，the resistivity of Mg co-doped ZAO thin films increases with the Mg doping content.