We adopt the post-additional thermal annealing (PATA) process to optimize the performance of the polymer solar cells (PSCs) with an active layer composed of a blend of regioregular poly (3-hexythiophene) (RR-P3HT) and fullerenes. It is found that compared with general annealing process, the crystallinity of RR-P3HT by PATA is enhanced, and the absorption peak is raised obviously at ~500 nm after PATA. With the optimized annealing conditions, the device shows an enhancement of 31% in short circuit current density, 5% in open circuit voltage (Voc), and 11% in the power conversion efficiency (PCE) compared with that of the general annealing device.