In this work, we used long (for a total of 2.5 µs) "all-atom" molecular dynamics (MD) simulations to investigate the adsorption behaviors of polyethylenimine (PEI) linear chains in Bis(2-ethylhexyl) sulfosuccinate sodium salt (or Aerosol-OT, AOT) reverse micelle (RM) in hexane with a water/AOT molar ratio (Wo) = 30. In absence of PEI chains, the RM is nearly spherical with a micelle radius of ~3.7nm. It takes less than 60 ns for that the PEI starts to unfold and adsorbs in the RM. After 450 ns, the completely unfolded PEI chains are localized at the AOT headgroup region with its nitrogen atoms interacting with the negatively charged headgroup of the AOT. A detailed analysis of the structure of the RM with the PEI reveals that the presence of the polymer chains does not affect significantly the size and shape of this RM with large Wo value which remains close to the results obtained from the MD of the PEI-free RM.