Doping of specific boron species in carbon frameworks plays an important role in enhancing the electrochemical properties of carbon. Herein, we prepared a tailored boron species-doped carbon nanotube (CNT) catalyst as the counter electrode (CE) of quantum dot sensitized solar cells (QDSSCs) by hydrothermal post-treatment of pre-oxidized CNTs. X-ray photoemission spectroscopy results show that boron is mainly present as oxygen-containing (C–O–B) and oxygen-free (C–B) boron in the sp² carbon framework, so different levels of pre-oxidation treatments can control the content of oxygen-containing boron species and thus achieve controlled doping of boron species. Electrochemical impedance spectroscopy shows that the optimized boron-doped CNTs (BCNT1) obtained a charge transfer resistance (R_ct) as low as 2.17 Ω, indicating excellent reduction activity to S_n²⁻ at the CE/electrolyte interface. The QDSSC based on the BCNT1 CE and CdS/CdSe photoanode achieves a power conversion efficiency of up to 4.55%, which is 3.10 times that of the pristine CNT CE. Our report confirms that tailorable boron doping is an effective strategy to enhance the activity of CNTs for S_n²⁻ reduction.