Nonsequential double ionization (NSDI) of Ar atom below the recollision threshold is investigated using the three-dimensional classical ensembles. The calculated results reveal a dominance of events for electron emission into opposite hemispheres, a clear minimum of the correlated electron-electron momentum distributions at the origin and a single peak structure of the Ar2+ ions longitudinal momentum spectra near zero momentum. The momentum spectra of the Ar2+ ions agree with the experimental results 101 053001 (2008) quantitatively. Trajectory back analyses show that the microscopic physical mechanisms of Ar NSDI are distinct for different intensities of the laser. When laser intensity equal to 0.7×1014 W/cm2, one recollision dominates the process of the recollision. However,the domination convert to multiple recollisions as the laser intensity decreases to 0.4×1014 W/cm2. In addition, the Coulomb attraction between the ion and electron plays an important role in microscopic dynamics of the electron in the process of Ar NSDI and eventually influences the final-state correlated electron-electron momentum distributions strongly.