To investigate the effect of icariin (ICA) on ovarian structure and function in rats with early-onset ovarian insufficiency (POI) and its possible mechanism. Methods Ten SD female rats with normal estrous cycle were randomly selected as the control group. The remaining rats were injected intraperitoneally with cyclophosphamide to prepare the POI model. The model rats were randomly divided into model group, ICA low (15 mg·kg-1), medium (30 mg·kg-1) and high (60 mg·kg-1) dose groups. The rats in the control group and model group were gavaged with the same amount of normal saline every day for 28 days. The wet weight and ovarian index of rats in each group were calculated. The histological structure of ovary was observed by HE staining. TUNEL staining was used to detect the apoptosis rate of granulosa cells. The levels of serum follicle stimulating hormone (FSH), estradiol (E2), anti mullerian hormone (AMH), malondialdehyde (MDA) and superoxide dismutase (SOD) were detected by ELISA. RT-qPCR was used to detect the relative expression of phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt), mammalian rapamycin target protein (mTOR) and ribosomal S6 protein kinase 1 (S6K1). The activity levels of PI3K, Akt, mTOR and S6K1 proteins and the relative expression of B-cell lymphoma/leukemia-2 gene (Bcl2), Bcl2 related X protein (Bax) and cleaved caspase-3 protein were detected by Western blot. Results POI rat model was successfully induced. Compared with the control group, the wet weight of ovaries, ovarian index, number of mature follicles and corpus luteum, serum E2, AMH and SOD levels in the model group and low, medium and high dose ICA groups decreased, the number of atresia follicles, apoptosis rate of granulosa cells, serum FSH and MDA levels increased, ovaries shrank in varying degrees, follicular structure and morphology degenerated in varying degrees, and the relative expression of Bax and cleaved caspase-3 protein in ovarian tissue increased, the relative expression of Bcl2 protein decreased, and the PI3K, Akt, mTOR, S6K1 mRNA and activities of protein decreased (P＜0.05). Compared with the model group, the above indexes and factor levels of rats in low, medium and high dose ICA groups were reversed (P＜0.05). The effect of ICA was dose-dependent (P＜0.05). Conclusion ICA could increase serum E2 level, reduce FSH level, improve ovarian structure and ovarian reserve function in POI rats. Its mechanism may be related to the activation of PI3K/Akt/mTOR pathway