A new double-lattice model, which is capable of describing and predicting the equilibrium properties of binary polymer solutions, was proposed by modifying Hu＇s double-lattice model by introducing a new interaction parameter and simplifying the expression of the Helmholtz energy of mixing. Using the proposed model, we investigated phase behaviours of various binary polymer solutions. The modified double-lattice model successfully describes the phase behaviours of binary polymer solutions having an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST).