To investigate the effect of letrozole in decreasing the early-stage ovarian hyperstimulation syndrome (OHSS) occurrence during the luteal phase for patients of OHSS high-risk after oocyte retrieval.A total of 176 high-risk OHSS patients were randomly divided into two groups after oocyte retrieval. Patients in experiment group (n = 86) received 5 mg letrozole per day from the retrieval day and last for 5 days. Others in control group (n = 90) received placebo. The serum concentration of FSH, LH, estradiol (E2), progesterone (P) and vascular endothelial growth factor (VEGF) from the day of hCG injection to days after injection (5 days, 8 days, 10 days) were measured. And the incidence of moderate and severe OHSS was observed.The concentration of E2 on the indicated days (5 days, 8 days, 10 days after hCG injection) in experiment group and control group were (5 727±2 089) versus (11 826±4 281) pmol/L, (1 613±879) versus (7 925±3 507) pmol/L, (193± 90) versus (1 628±888) pmol/L; the concentration of VEGF on the indicated days in the two groups were (80±14) versus (108±19) ng/L, (66±11) versus (126±14) ng/L, (48±7) versus (148±14) ng/L; the concentration of E2 and VEGF were lower than those in control group (all P < 0.01). The FSH concentration in experiment group were (2.1±1.1) and (3.5±1.3) U/L on the day of fifth and eighth day after hCG injection, which were significantly higher than (0.7±0.3) and (0.7±0.4) U/L in control group (P < 0.05); the LH concentration in experiment group were (0.26±0.19) and (0.72±0.60) U/L on the day of fifth and eighth day after hCG injection, which were significantly higher than (0.11±0.03) and (0.14±0.08) U/L in control group (P < 0.05). The incidence of moderate and severe OHSS was signicantly decreased after letrozole treatment compared with control group [2% (2/86) versus 12% (11/90), P < 0.05].Administration of 5 mg/d letrozole for 5 days during the luteal phase can reduce the E2 and VEGF levels for the high-risk OHSS patients who needed cryopreserve all embryos, and also reduce the occurrence of early OHSS.