Since the observation of the enhanced double ionization of Helium atoms at a moderate laser intensity, much attention has been concentrated on exploring the physical mechanism of nonsequential double ionization (NSDI) of atoms in strong laser fields[1].To investigate NSDI and understand its mechanism, we developed a 3D semi-classical two-electron model including all the interactions, i.e., Coulumb interaction of ion and electrons, interaction between electron and electron, and the electric field of the laser.In this model one electron is released at the outer edge of the field-suppressed Coulomb barrier through tunneling with a rate given by the ADK theory, the bound electron is sampled from a microcanonical distribution, and the subsequent evolution of the two electrons with the these initial conditions is governed by Newtons equations of motion.With the model, numerical calculations reproduce the excessive double ionization and the photoelectron spectra observed experimentally both quantitatively and qualitatively for Helium [2], and found correlated electron emission for NSDI of Helium[3].Recently, we developed our model to study the double ionization for circularly polarized laser fields and low laser intensities, and high-Z atoms as Argon.For circularly polarized laser fields case, a velocity window for recollision to occur is found, and the model reproduces the experimental results for magnesium and explains the apparently conflicting experimental results in terms of an analytical formula that demarcates the phase diagram for the NSDI[4].According to the semiclassical model we also found that the doubly excited states are largely populated after the laser-assisted recollision and large amounts of double ionization dominantly takes place through sequential ionization of doubly excited states at a low laser intensity[5].Moreover, for High-Z atoms, we showed that the recollision induced excitation and the shielding effect are very important for strong-field double ionization and the electron momentum spectra are lacking the signature of the side-by-side and back-to-back emission which reveal a remarkable noncorrelation behavior[6].