A stable morphology in the photoactive layer is a prerequisite for increasing the lifetime of organic solar cells. Intense research efforts focusing on this research topic have typically resorted to complicated synthetic methods to reach this goal. Herein, the authors present a facile approach to directly achieve efficient polymer solar cells with a remarkably enhanced thermally stable morphology by constructing densely distributed poly(3-hexylthiophene) (P3HT) nanofibers in the pristine composite films with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) from solution without any post treatments. Controlled experiments reveal that the presence of numerous preformed P3HT nanofibers in the pristine films, with much larger size than P3HT and PCBM molecules, provides a fixed and rigid network to spatially confine the diffusion of PCBM molecules during thermal annealing, thus preventing the formation of large-scale PCBM crystals. This simple method represents a “one-step” way to prepare high performance photovoltaic devices with thermally stable morphologies and no necessary post treatments.