〔Abstract〕 Objective To study the effect of changes in estradiol(E2) levels on the reproductive outcomes of patients with polycystic ovary syndrome(PCOS) undergoing controlled ovarian stimulation(COS) with gonadotropin-releasing hormone antagonist(GnRH-ant) protocol. Methods A retrospective study was conducted involving 338 patients with polycystic ovary syndrome(PCOS) infertility who underwent GnRH-ant protocol for controlled ovarian hyperstimulation followed byin vitrofertilization(IVF) or intracytoplasmic sperm injection(ICSI), and subsequently underwent their first frozen-thawed embryo transfer(FET). The clinical data of these patients were analyzed.Patients were grouped based on the changes in serum E2levels on the first and fourth day of GnRH-ant administration(blood samples collected before GnRH-ant injection): the E2elevation group(Group A, E2value increased more than 30%, 165 cases), the E2stable group(Group B, E2rate of change was within-30%～30%, 162 cases), and the E2decline group(E2value declined more than 30%, 11 cases, not included in statistical analysis due to small sample size).The differences in demographic characteristics, ovulation induction outcomes, embryo outcomes and clinical pregnancy-related indicators were analyzed between Groups A and B. Results There were no statistically significant differences in basic information such as age, duration of infertility, body mass index(BMI), basal endocrine levels, and anti-Müllerian hormone(AMH) levels between groups A and B. Regarding embryo characteristics, there were no statistically significant differences in fertilization method and number of transferred embryos between groups A and B. However, group A had a higher total number of retrieved oocytes, normal fertilization rate, number of high-quality embryos, and rate of high-quality embryos compared to group B, with statistically significant differences(P<0.05). In terms of clinical pregnancy outcomes, there were no statistically significant differences in the incidence of severe ovarian hyperstimulation syndrome(OHSS) between groups A and B. However, group A had higher rates of clinical pregnancy, implantation, and live birth compared to group B, with statistically significant differences. Group A also had a lower rate of early miscarriage compared to group B, with statistically significant differences. Conclusion Choosing the GnRH-ant protocol for IVF/ICSI-FET in PCOS patients, if the blood E2level increases by more than 30% after 4 days of adding the antagonist(blood sample collected before administering GnRH-ant), the clinical pregnancy outcome will be better.