Polycystic ovary syndrome (PCOS) is a common endocrine disorder affecting women of reproductive age. Considerable research has confirmed that afamin is associated with the prevalence and development of metabolic syndrome. Thus, this study investigated the mechanism of action of afamin in PCOS and its potential therapeutic value. We found that PCOS patients had higher levels of afamin than normal control subjects. Afamin significantly enhanced the overall antioxidant ability and activity of antioxidant enzymes and reduced the levels of 8-hydroxydeoxyguanosine (8-OHdG), malondialdehyde (MDA), reactive oxygen species (ROS) and superoxide anion (O₂^.-) in human ovarian granulosa tumor cells (KGN cells). In addition, afamin was also found to protect KGN cells against testosterone propionate (TP)-induced mitochondrial damage and apoptosis. Additionally, silencing of SIRT1 revealed that SIRT1 protected KGN cells against TP-induced oxidative stress (OS) injury, mitochondrial damage and apoptosis. Furthermore, this study showed that besides restoring the estrous cycle in PCOS mice, afamin might also reduce the increased cycling testosterone (T) and luteinizing hormone (LH) levels and LH/Follicle-stimulating hormone (FSH) ratio, as well as decrease the number of cystic follicles, indicating the significance of afamin in the treatment of PCOS. Moreover, afamin reduced oxidative damage in PCOS mice by enhancing their antioxidant capacity. Also, afamin may protect KGN cells against TP-induced OS by enhancing their antioxidant ability, restoring mitochondrial function, and inhibiting cell apoptosis by upregulating the expression of SIRT1. Thus, afamin may play a protective role in PCOS mice.