Contradictory experimental absorption spectra blue shift and red shift results have been reported in the literatures. To solve this problem, this study investigates the electronic structure and absorption spectra of Zn1-xCuxO (x= 0, 0.0313 and 0.0625, respectively) and Zn32CuO32 supercells employing first-principles calculations with GGA+U method. By increasing the Cu substitutional doping concentration from 3.13% to 6.25%, the following results are obtained: increased magnetic properties, narrower band gaps, and a significant red shift in the absorption spectrum. These findings are in good agreement with the experimental results. The changes of band gap and absorption spectrum for interstitial doping and substitutional doping are opposite. (C) 2014 Elsevier B.V. All rights reserved.