Objective: To evaluate oncological and reproductive outcomes of women ≤40 years undergoing fertility-sparing surgery (FSS) for stage Ⅱ or Ⅲ borderline ovarian tumor (BOT). Methods: The patients with BOT and ≤40 years old with stage Ⅱ-Ⅲ BOT who underwent FSS enrolled from the First Affiliated Hospital of Zhengzhou University between January 2011 and March 2023 were analyzed retrospectively. The clinical data and follow-up results were obtained and analyzed. The univariate and multivariate Cox proportional hazard regression analysis were used to explore high-risk factors associated with prognosis. Additionally, pregnancy outcomes were also analyzed. Results: (1) A total of 79 patients with stage Ⅱ-Ⅲ BOT who have been treated with FSS were conducted, with an average age of (27.5±6.7) years old. The median tumor maximum diameter were 10.4 cm (range: 4.8-90.0 cm). The International Federation of Gynecology and Obstetrics (FIGO) stage was stage Ⅱ in 45 cases and stage Ⅲ in 34 cases. According to the pathological types, there were 48 cases of serous tumor, 21 cases of mucinous tumor, 1 case of endometrioid tumor, and 9 cases of mixed types. There were 41 cases of unilateral ovarian involvement, 38 cases of bilateral ovarian involvement. There were 5 cases of microinvasion, 17 cases of micropapillary subtype. Extra-ovarian invasive implants were found in 5 cases, and there were 31 cases of merged ascites. (2) Tumor outcomes: the median follow-up time from primary cytoreduction were 58 months (range: 8-146 months). At the end of the observation period, 24 cases (30%, 24/79) recurred, among them 5 cases had two recurrences and 2 cases had three recurrences. There were 2 cases (3%, 2/79) of death and 1 case (1%, 1/79) of survival with tumor. (3) Analysis of prognostic risk factors: the results of univariate analysis showed that mucinous tumor, tumor maximum diameter >13.15 cm, FIGO stage Ⅲ, merged ascites, micropapillary subtype, invasive implantation, and bilateral ovarian involvement were independent risk factors (all P<0.05) for disease-free survival (DFS). FIGO stage Ⅲ (HR=4.555, 95%CI: 1.525-13.607; P=0.007) and micropapillary subtype (HR=2.396, 95%CI: 1.003-5.725; P=0.049) were found to be related to DFS through the multivariable Cox proportional hazard regression analysis. (4) Pregnancy outcomes: among the patients with fertility intentions 36 cases (46%,36/79), 29 cases (81%, 29/36) had successful pregnancies, and 27 cases (75%, 27/36) had successful births. Conclusions: Patients with stage Ⅱ-Ⅲ BOT underwent FSS have favorable survival and pregnancy rates. Micropapillary subtypes and FIGO staging (stage Ⅲ) are the significant risk factors of DFS.