We have investigated the structure parameters and electronic properties of Cd1-xZnxS doped with alkaline earth metals by first-principles calculations on the basis of density functional theory to predict the photocatalytic activity for hydrogen generation. The optimized structure parameters show that the smaller dopants tend to pull inward their surrounding atoms and the larger ones tend to push them outward. The negative defect formation energies indicate that almost all of the doped Cd1-xZnxS are stable. Be or Mg doping raises the conduction band edge, and the d orbital of Ca, Sr, Ba interacts with the S-3p orbital leading to the changes of valence band. The band gap of the doped systems decreases with the increase of atomic number of dopant, and so alkaline earth metal doping acts as an effective method to adjust the optical properties. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.