A model for the Gibbs energy of mixing for the systems composed of water, neutral polymer and polyelectrolyte is established by combining the theories of Flory-Huggins and Manning, and by treating the free counterions as complementary species. The phase behaviour of two model systems and its dependence on the salt concentration are studied by using the model. The basic features of the phase behaviour of the systems can be captured, and the calculated results are in good agreement with the experimental data.