Organic solar cells (OSCs) are a promising cost-effective alternative for utility of solar energy, and possess advantages such as low cost, light weight, and flexibility. Much attention has been focused on the development of OSCs (synthesis of new donor or acceptor materials, control of morphology of active layer and fabrication of new device structures) which have seen a dramatic rise in efficiency over the last decade. Since the bulk heterojunction concept was reported in 1995, two-component (binary) blend has been predominant as active layer and achieved great success with power conversion efficiencies (PCEs) up to ca. 10%. Ternary blend of active layer consisting of a donor material, an acceptor material and a third component has received increasing attention in recent five years. The third components include light-absorbing polymers or small molecules, fullerene or non-fullerene acceptors, inorganic nanomaterials (quantum dots, metal nanomaterials or carbon-based nanomaterials) and organic nonvolatilize additives (polymers or small molecules). Compared with binary blend, ternary blend may present some advantages: broader and stronger absorption; more efficient charge transfer and charge transport; better morphology and improved stability. In this work, we fabricated ternary blend OSCs based on P3HT/TT-TTPA/PC61BM blend. TT-TTPA is a conjugated small molecule with thiazolothiazole as acceptor unit, triphenylamine as donor unit and thiophene as bridge. TT-TTPA has good miscibility with PC61BM; the phase separation scale of TT-TTPA/PC61BM blend is very small. During solvent annealing and thermal annealing, the small amount of TT-TTPA in P3HT domains can move from P3HT domains to PC61BM domains, thus increasing the phase purity of P3HT domains that can undergo crystallization. After optimization of the weight ratio of TT-TTPA in ternary blend, we achieved better PCE (4.41%) relative to binary blend (3.85%). Effects of TT-TTPA on absorption, crystallinity and morphology of P3HT:PC61BM blend films were investigated by UV-vis, X-ray diffraction and atomic force microscopy.