Zn1-xMgxO thin films have been prepared by sol-gel spin-coating technique instead of the CdS buffer layers synthesized by conventional chemical bath deposition for Cu(In,Ga)Se2 (CIGS) solar cells. The effects of Mg doping amount on the structural, morphological, optical properties of Zn1-xMgxO films and band alignment of Zn1-xMgxO/CIGS are investigated by X-ray diffraction, Atmoic force microscope, UV-vis-NIR spectroscopy and X-ray photoelectron spectroscopy. The results show that these properties changed with the increase of Mg. All the films are amorphous. The surface morphology changes from stripe to hexagon nanoparticles and the surface roughness decreases from 23.53 nm to 1.14 nm. The optical band gap increases from 3.55 eV to 3.62 eV and conduction band offsets changes from +0.68 eV to -0.33 eV. Accordingly, the band alignment of Zn1-xMgxO/CIGS changes from “spike-like” to “cliff-like”. An efficiency of up to 5.83% is achieved for a CIGS solar cell with a Zn1-xMgxO (using a molar ratio 0.1 of the Mg source and the Zn source in the precusor solution) buffer layer, which is attributed to the optimized conduction band offset of +0.22 eV at the Zn1-xMgxO/CIGS interface.