Eco-friendly polymer solar cells (eco-PSCs) based on aqueous-soluble conjugated materials have recently received great research attention. In this work, we report the development of an aqueous-processable C70-based fullerene derivative (PC71BO15) for eco-PSCs processed by water/ethanol co-solvents. The desirable aggregation behavior and enhanced light absorption ability of PC71BO15 have enabled the fabrication of PPDT2FBT-A:PC71BO15-based eco-PSCs with PCEs of up to 2.51%, which is the highest value reported to date for aqueous-processed PSCs. These PPDT2FBT-A:PC71BO15 eco-PSCs exhibit significantly higher PCEs than those of the reference PPDT2FBT-A:PC61BO15 devices over all device-processable water/ethanol compositions. At the optimal water/ethanol composition (v/v = 15:85), the PCE of the PPDT2FBT-A:PC71BO15 eco-PSCs is 73% higher than that of the PC61BO15-based counterparts, as a result of enhanced light absorption. Importantly, the PC71BO15-based eco-PSCs show much higher tolerance in their PCEs to the water/ethanol composition. For example, the PCEs of the PPDT2FBT-A:PC71BO15 eco-PSCs at a 30:70 water/ethanol ratio maintain 89% of the optimal performance at a 15:85 ratio, whereas only 45% for PC61BO15-based devices. This large difference in terms of water-tolerant behavior is mainly related to different aggregation behaviors between PC71BO15 and PC61BO15 in blend film, which was carefully investigated by electrical, optical and morphological characterizations. This high water composition tolerance affords excellent reproducibility of the PC71BO15-based eco-PSCs with eco/human-friendly aqueous processing in ambient conditions.