Objective To study the effect of changes in estradiol(E2)levels on the reproductive outcomes of pa-tients 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 by in vitro fertilization(IVF)or intracytoplasmic sperm injection(ICSI),and subse-quently underwent their first frozen-thawed embryo transfer(FET).The clinical data of these patients were ana-lyzed.Patients were grouped based on the changes in serum E2 levels on the first and fourth day of GnRH-ant ad-ministration(blood samples collected before GnRH-ant injection):the E2 elevation group(Group A,E2 value in-creased more than 30％,165 cases),the E2 stable group(Group B,E2 rate of change was within-30％～30％,162 cases),and the E2 decline group(E2 value declined more than 30％,11 cases,not included in statistical a-nalysis 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 trans-ferred 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 sta-tistically 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 E2 level increases by more than 30％after 4 days of adding the antagonist(blood sample col-lected before administering GnRH-ant),the clinical pregnancy outcome will be better.