By using the three-dimensional classical ensemble model,the recollision dynamics in nonsequential double ionization(NSDI) of Ar by 780-nm laser pulses at (6-1.2) × 1014 W/cm2 was extensively studied.We revealed the picture of multiple-recollision in the double ionization events at the laser intensity region below the recollision-ionization threshold.Via tracing the NSDI trajectories,it was found that the contribution of these multiple-recollision events increases as the laser intensity decreases.In this low intensity region,many multiple-recollision induced NSDI trajectories occur through the doubly excited states.The decay speed of the doubly excited state decreases with the decreasing laser intensity.