The organic semiconductors of poly(3-hexylthiophene) (P3HT), poly(3-octylthiophene)(P3OT), poly(3-dodecylthiophene) (P3DDT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were used to make active layers. The active layers of an organic thin film solar cell were fabricated by the P3HT/PC71BM, P3HT/P3OT/PC71BM and P3HT/P3DDT/PC71BM fluid, respectively. Compared with the performances, the performance of the organic solar cell fabricated by P3HT/P3OT/PC71BM thin film indicates the best. The open circuit voltage is 0.61 V. The short-circuit current density is 6.55 mA cm2. The fill factor is 0.51. The power conversion efficiency is 2.07%.