A relativistic mean field model is used to study the ground-state properties of neutron-rich nuclei in Ca isotopes.An additional isoscalar and isovector nonlinear coupling has been introduced in the relativistic mean field model,which could soften the symmetry energy, while keep the agreement with the experimental data. The sensitivity of proton and neutron density distributions and single particle states in Ca isotopes to the additional isoscalarisovector nonlinear coupling term is investigated. We found that the binding energies, the density distributions of single particle levels are strongly correlated with the density dependence of the symmetric energy in nuclear matter.