The predominant method to prepare polymer membranes is based on phase inversion. However, this method always leads to a dense skin with low porosity when normal polymers are used. Using the self-assembly of certain block copolymers, it is possible to prepare uniform pores with high porosity, but the prices of these polymers are too high to be afforded in practical applications. Here, we report a novel strategy to prepare highly porous and asymmetric polymer membranes using the widely used poly(vinylidene fluoride) (PVDF) as a prototype. The method combines spinodal decomposition with phase inversion utilizing mixed solvents that have the unique upper critical solution temperature phase behavior. The spinodal decomposition generates a thin surface layer containing a high density of relatively uniform pores in the mesoporous range, and the phase inversion generates a thick bulk layer composed of macrovoids; the two types of structures are interconnected, yielding a highly permeable, selective, and mechanically strong porous membrane. The membranes show an order of magnitude higher water permeance than commercial membranes and efficient molecular sieving of macromolecules. Notably, our strategy provides a general toolbox to prepare highly porous membranes from normal polymers. By blending PVDF with cellulose acetate (CA), a highly porous PVDF/CA membrane was prepared and showed similarly high separation performance, but the higher hydrophilicity of CA improved the membrane flux in the presence of proteins.