The nonsequential double ionization (NSDI) of Mg atoms below the recollision threshold laser power density is investigated based on the classical ensemble model. For a few-cycle linearly polarized laser pulses with a power density of 3.0×10 13 W·cm -2, the electron pairs in the correlated-electron momentum distribution at a final state mainly locate in the first, second and fourth quadrants. Moreover, the electron pairs in the first quadrant show an obvious correlation behavior. By analyzing the delay time between the recollision and the double ionization, it is found that the different delay time corresponds to different ionization processes and the delay time plays an important role in the process of the electron emission. For the NSDI events with delay time less than a half cycle, two electrons tend to emit with an opposite direction, however, for those with delay time larger than a half cycle, there exist emission electrons with the same direction.