Infertility patients who frozen all embryos due to the risk of OHSS were randomized allocated to letrozole or polygeline injection group. Inclusion criteria meet one of the following conditions: (1) oocyte is more than or equal to 20; (2) human chorionic gonadotropin (hCG) injection on serum estradiol levels greater than or equal to 5000 pmol/L; (3) on the day of oocyte unilateral or bilateral ovarian diameter greater than or equal to 10 cm (4) follicle puncture is larger than or equal to number 20. After the informed consent was signed, letrozole or polygeline injection was randomized allocated after oocyte retrieval. OHSS was determined according to Golan diagnosis standards.