  The dependence between neutron-to-proton yield ratio ($R_{np}$) and neutron
skin thickness ($\delta_{np}$) in neutron-rich projectile induced reactions is
investigated within the framework of the Isospin-Dependent Quantum Molecular
Dynamics (IQMD) model. The density distribution of the Droplet model is
embedded in the initialization of the neutron and proton densities in the
present IQMD model. By adjusting the diffuseness parameter of neutron density
in the Droplet model for the projectile, the relationship between the neutron
skin thickness and the corresponding $R_{np}$ in the collisions is obtained.
The results show strong linear correlation between $R_{np}$ and $\delta_{np}$
for neutron-rich Ca and Ni isotopes. It is suggested that $R_{np}$ may be used
as an experimental observable to extract $\delta_{np}$ for neutron-rich nuclei,
which is very significant to the study of the nuclear structure of exotic
nuclei and the equation of state (EOS) of asymmetric nuclear matter.
