We investigate the effects of H2O molecule environment on the transport properties of a seven-atom carbon wire coupled to two Al(100) electrodes based on a recently developed ab initio nonequilibrium Green function formalism. Our results show that the transport properties are sensitive to the variation of the number and the position of the H2O molecule on the carbon wire. Especially, the equilibrium conductance of the carbon wire with single H2O molecule exhibits an oscillatory behavior with the shift of positions of the H2O molecule. For the case of two H2O molecules, the contribution of the third eigenchannel becomes larger in some configurations. The calculated current-voltage curves show different behaviors with the variation of the positions of the H2O molecules. In certain cases, large negative differential resistance is found, but not in other cases.