This study is the first to report highly efficient production of homogeneous large-scale thin polymer films with a sponge-like porous structure via an improved phase separation method. A mixture of tetrahydrofuran and methanol, which is used as a highly volatile solvent/nonsolvent pair, is crucial to induce phase separation and thus determines the resulting structure. The obtained porous structure is composed of intermingled sticks in a size range of 50-100 nanometers and bumps with a diameter of about 200 nanometers, revealing a morphology that resembles a bird's nest. The pore size and porosity of the porous film are finely controlled by varying the mixture composition. Furthermore, by employing a water-soluble interlayer before the phase separation process, the obtained porous film, even with extremely large area, can be easily detached from solid substrates without damaging the film. Notably, it possesses a skin-free layer at the bottom side where it contacts the interlayer surface. The formation mechanism of the sponge-like porous structure was studied and it was found that the progressive enrichment of methanol during the evaporation process is favorable for absorbing water vapor from the air into the solution, which is responsible for phase separation. In addition, a water-miscible mixture of solvent and nonsolvent, which can delay the liquid-liquid demixing process, is essential for the formation of the sponge-like porous structure. (C) 2016 Elsevier Ltd. All rights reserved.