Recently, ternary bulk-heterojunction (BHJ) polymer solar cells (PSCs) occur as an attractive strategy with simple fabrication technology to extend the spectrum of wide bandgap polymers into the near infrared region by adding a narrow bandgap sensitizer. In this paper, a series of cells including binary BHJ-PSCs with P3HT:PCBM as the active layer (control cell) and ternary BHJ-PSCs with different PTB7-Th doping concentrations are fabricated to investigate the effect of PTB7-Th on the performance of PSC. The short-circuit current density (Jsc) and fill factor (FF) of the ternary PSCs are simultaneously improved by adding a small amount of PTB7-Th into P3HT:PCBM. The champion photoelectric conversion efficient of ternary PSCs (with 15 wt% PTB7-Th) is 3.71%, which is larger than 2.71% of the control cell. In a ternary device, the absorption region shows a distinct red-shift and the relative absorption intensity from 650 nm to 800 nm is gradually enhanced with the incrtease of PTB7-Th doping concentration. The increased photon harvesting in the solar spectral range results in an increased short-circuit current density. However, despite the fact that the photoluminescence (PL) spectrum of P3HT has a large overlap with the absorption spectra of PTB7-Th, which makes it possible for Frster resonance energy to transfer between P3HT and PTB7-Th, the PL intensity of P3HT at 650 nm is quenched with the increase of PTB7-Th doping concentration while the photoluminescence remains almost the same in the long wavelength region, which suggests that the main mechanism between PTB7-Th and P3HT is photo-induced electron transfer from P3HT to PTB7-Th (hole transfer from PTB7-Th to P3HT), not energy transfer. The PSCs with P3HT:PTB7-Th (1:1) as an active layer display a large Jsc compared with the P3HT-based one. When the concentration of PTB7-Th decreases and the concentration of P3HT is unchanged (P3HT:PTB7-Th 1 : 0.5), the Jsc can be further enhanced. The increased Jsc value of P3HT: PTB7-Th (1:0.5) PSCs confirms that the photo-generated excitons can be dissociated into free charge carriers at the P3HT:PTB7-Th interface and reinforce the charge transfer between P3HT and PTB7-Th. In P3HT:PCBM binary organic solar cell, the photo-generated excitons only can be directly dissociated into free charge carriers at the P3HT:PCBM interface and then transported to the respective electrodes, while incorporating PTB7-Th, the interaction between P3HT and PTB7-Th also makes the photo-generated excitons dissociated at the interface of P3HT:PTB7-Th, and at the interface of PTB7-Th:PCBM. The increasing of excitons dissociated leads to a higher FF. The present study is the first report on utilizing PTB7-Th in P3HT:PCBM PSC.