Using the dynamic density functional method, we studied the structural evolution of a thin film of polymer blends placed between the solid substrate and free surface. We observed various types of structural instabilities such as the spinodal wave and the roughening of the free surface due to droplet formation. We proposed a simple theoretical argument based on the Neumann triangle condition among the interfacial tensions to construct a phase diagram for the instability of the free surface and confirmed this by a series of dynamic density functional simulations.