The influences of substrate temperature and sputtering pressure on microstructure and optical properties of ZnO thin film buffer layer deposited by magnetron sputtering were investigated. The results show that the substrate temperature and the sputtering pressure have a great effect on the surface morphology, average grain size, forbidden band width and optical transmittance of ZnO buffer layer. Through the comprehensive analysis, it is concluded that the optimal ZnO buffer layer film can be deposited at substrate temperature of 250 oC and sputtering pressure of 0.6 Pa. Under this technical condition, ZnO buffer layer film presents well (002) preferred orientation in c axis, compact and uniform structure with the forbidden band width of 3.24 eV and average visible light transmittance of 86.93%, satisfying the requirement of the buffer layer of CIGS solar cell