Phase diagrams were calculated based on Flory-Huggins solution thermodynamics to investigate the effects of polydispersity of polymer molecules and interaction parameter on the phase equilibria of crystallizable polymer solutions. The polydispersity was modeled with blends of two monodisperse polymers differing in chain lengths as a simplification. It was found that a longer chain length component could be separated easily to a polymer-rich phase by liquid demixing, but a shorter chain length component might exist at relatively constant concentration in each phase on on fractionation. The influence of polydispersity on the liquid-solid phase equilibrium was small, and the phase boundary could be moved significantly in the region of low concentration of polymer by a small change of temperature. Liquid-Liquid phase separation was more sensitive to the interaction between polymer and solvent than Liquid-solid phase transition. Numerical calculations showed that the temperature at which liquid-liquid phase separation was coupled with liquid-solid phase equilibrium increased with a lower concentration of the polymer due to polydispersity of polymer chain lengths, and this phenomenon was observed at a lower temperature with more favorable interaction. The results were consistent with the experimental observations of isotactic polypropylene solutions.