A simple general thermodynamic model is presented for predicting the shear-induced phase changes in polymer fluids. It is based on the established concept of generalized Gibbs free energy of mixing, with an extra entropic storage term due to flow. The latter term is related to the conformational changes of the polymer chains due to flow, via the first normal stress difference or the viscoelastic storage modulus. The general energy analysis is carried out within the framework of Flory＇s statistical mechanical lattice model. As such, this approach provides some insights with respect to the configurational changes of a polymer fluid subject to shear flow, and thus to the molecular origins of shear-induced structural changes. Moreover, the model is universal with respect to its applicability to different kinds of polymeric fluids, such as solutions and blends. Theoretical predictions of the phase diagrams under shear agree reasonably well with selected experiments with the systems polystyrene/dioctyl phthalate and poly(styrene-co-maleic anhydride)/poly(methyl methacrylate).