The efficiency of polymer solar cells (PSCs) can be essentially enhanced by improving the performance of electron-acceptor materials, including by increasing the lowest unoccupied molecular orbital (LUMO) level, improving the optical absorption, and tuning the material solubility. Here, a new soluble C70 derivative, dihydronaphthyl-based C70 bisadduct (NC70BA), is synthesized and explored as acceptor in PSCs. The NC70BA has high LUMO energy level that is 0.2 eV higher than [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and displays broad light absorption in the visible region. Consequently, the PSC based on the blend of poly(3-hexylthiophene) (P3HT) and NC70BA shows a high open-circuit voltage (Voc = 0.83 V) and a high power conversion efficiency (PCE = 5.95%), which are much better than those of the P3HT:PCBM-based device (Voc = 0.60 V; PCE = 3.74%). Moreover, the amorphous nature of NC70BA effectively suppresses the thermally driven crystallization, leading to high thermal stability of the P3HT:NC70BA-based solar cell devices. It is observed that the P3HT:NC70BA-based device retains 80% of its original PCE value against thermal heating at 150 °C over 20 h. The results unambiguously indicate that the NC70BA is a promising acceptor material for practical PSCs.