Crystallization-induced vertical stratified structures were constructed based on double-crystalline poly(3-hexylthiophene) (P3HT)/poly(ethylene glycol)s (PEG) systems at room temperature, in which the P3HT crystallinity and the mechanism were investigated. Vertical stratified microstructures with highly crystalline P3HT network on the surface were formed when depositing from marginal solvents, while lateral phase-separated structures or low P3HT crystallinity were observed for good solvents. The morphological differences came from the solvent effect. In marginal solvents, p-xylene and dichloromethane, P3HT large-scale microcrystallites were generated in solution, which ensured the priority of P3HT crystalline sequence, and phase separation began in the liquid states. When the PEG matrix began to crystallize, great energy from which the second phase separation was induced drove P3HT crystallites to the surface, resulting in the formation of vertical stratified microstructures with highly crystalline P3HT network on the surface. The method, crystallization-induced phase segregation of crystalline crystalline blends in marginal solvent, provides a facile way to construct vertically stratified structures, in which P3HT highly crystalline network is favored.