A three-dimensional classical micro-canonical model is used to investigate the ion momentum distributions of Ar atom in nonsequential double ionization with a few-cycle laser pulse. It is found that the momentum distribution has a strong dependence on the carrier-envelope phase of the few-cycle pulse. The centre of gravity of the asymmetry distribution shifts from negative to positive ion momenta with carrier-envelope phase increasing and it is reversed when the carrier-envelope phase changes by π. The momentum distributions of the Ar2+ ion show excellent agreement with the experimental results when we take into account the change in Coulomb potential between the nucleus and electron in the recollision process.