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The intrinsic stability of the acceptor is a crucial component of the photovoltaic device stability. In this study, we investigated the efficiency and stability of the nonfused-ring acceptors LC8 and BC8 under indoor light conditions. Interestingly, we found that devices based on BC8 with terminal side chains exhibited a higher indoor efficiency and stability. Through accelerated aging experiments, we discovered that the acceptors generate singlet oxygen under light exposure with BC8 demonstrating lower levels of ROS compared to LC8. We attribute this difference to the modulation of the acceptor aggregation orientation. Furthermore, the generated reactive oxygen species (ROS) further deteriorate the acceptor structure, and this phenomenon is also observed in high-efficiency acceptor structures, such as Y6. Our research reveals important mechanisms of acceptor photo-oxidation processes, providing a theoretical basis for enhancing the intrinsic stability of acceptors.