  An improved Quantum Molecular Dynamics model is proposed. By using this
model, the properties of ground state of nuclei from $^{6}$Li to $^{208}$Pb can
be described very well with one set of parameters. The fusion reactions for
$^{40}$Ca+$^{90}$Zr, $^{40}$Ca+$^{96}$Zr and $^{48}$Ca+$^{90}$Zr at energy near
barrier are studied by this model. The experimental data of the fusion cross
sections for $^{40}$Ca+$^{90,96}$Zr at the energy near barrier can be
reproduced remarkably well without introducing any new parameters. The
mechanism for the enhancement of fusion probability for fusion reactions with
neutron-rich projectile or target is analyzed.
