Water adsorption and decomposition on the Co(0001)surface has been systematically studied by spin-polarized density functional theory(DFT)calculations carried out with the program package Dmol3 in Materials Studio and atomic thermodynamics.H2O adsorption mechanism has been analyzed by partial density of states(PDOS).The formation of O-Co bond is caused by overlapping of Cos 3d and Os 2p orbitals.The possible structure of adsorbed H2O molecules comprised of monomer-hexamer have been investigated and the phase diagram shows that only two configurations are stable thermodynamically: clean Co(0001)surface and H2O hexamer adsorption.The interaction among adsorbed H2O molecules can help stabilize adsorption configuration by forming H bonds.The presence of adsorbed oxygen species has a great influence on the decomposition of water and can significantly reduce the activation barrier.