Abstract: Objective：To investigate the effects of different endometrial preparation protocols in frozen-thawed embryo transfer (FET) cycles on the clinical outcomes of the patients with polycystic ovarian syndrome (PCOS). Methods: The data of 633 FET cycles of PCOS patients from January 2016 to December 2017 were analyzed retrospectively. According to endometrial preparation protocols, the patients were grouped into the　ovulation induction group（group A，n=61）, the hormone replacement group（group B，n=347）and the pituitary down-regulation combined with hormone replacement group （ｇｒoup C，n=22５）． Ｔhe basic characteristics and the pregnancy outcomes were compared among three groups. Results: There were no significant differences in the age，time of infertility, body mass index, basic FSH level, E2 level, the number of transfered embryos，the rate of embryo implantation, the clinical pregnancy rate and the multiple pregnancy rate among three groups （all P＞0.05）. The basic level of LH in the group C was higher than that in the group B (P＜0.０５），ａnd endometrial thickness on the day of embryo transfer in the group C was higher than those in the group A and the group（both P＜0.05）. The levels of LH, FSH, and E2 before hormone administration in the group C were significantly lower than those in the group A and group B （ａｌl P＜0.0５）． The early abortion rate in the group A was significantly lower than those in the group B and the group C（P＜0.05），and the ectopic pregnancy rate in the group B was significantly lower than that in the group C（P＜0.05）. Furthermore, there were no significant differences in the duration and total dosage of estrogen administrated between the group B and the group C（P＞0.05）. Conclusions: For the patients with PCOS undergoing FET, the endometrial preparation protocols such as ovulation induction, hormone replacement and down regulation combined with hormone replacement can obtain satisfactory pregnancy outcomes，and the first two strategies have their own advantages in reducing the early abortion rate and the ectopic pregnancy rate.

Key words: Polycystic ovarian syndrome, In vitro fertilization, Embryo transfer, Pregnancy outcome