In order to study the surface of Ga1-xAlxN photocathodes theoretically, atomic structure, band structure, density of state, surface energy, work function and optical properties of Ga0.75Al0.25N(0 0 0 1) surface are calculated using the plane-wave pseudo-potential method based on the density function theory. It is found that the Ga0.75Al0.25N(0 0 0 1) surface is not flat, the Al atom is more close to N atoms while the Ga atoms are far away from N layer. The surface charges shift from bulk to surface by the effect of dipole moment, the charges of Ga atoms in the top-most layer decrease while that of Al atom change little. The surface energy and work function of Ga0.75Al0.25N(0 0 0 1) are 2.169 J/m(2) and 4.36 eV respectively. The absorption coefficient of Ga0.75Al0.25N(0 0 0 1) surface is lower than bulk Ga0.75Al0.25N. This provides a theoretical basis for the design and application of Ga1-xAlxN photocathodes. (C) 2013 Elsevier B. V. All rights reserved.