Background:There is no absolute consensus for the best time for triggering.The aim of this study was to investigate the effect of different proportion of dominant follicles (PDF) on the human chorionic gonadotropin (HCG) day for the clinical outcomes in patients with polycystic ovary syndrome (PCOS) of different ovarian stimulation protocols.Methods:A total of 371 cycles of the gonadotropin-releasing hormone (GnRH) agonist long protocol and 347 cycles of GnRH antagonist protocol from January 2014 to December 2016 were included in this retrospective study.Based on the PDF on the day of the HCG administration,the included patients were divided into three groups:Group A (low PDF),PDF ＜20％;Group B (medium PDF),20％≤ PDF ≤40％;Group C (high PDF),PDF ＞40％.The measurements regarding ovarian stimulation characteristics,fertilization rate,top quality embryo rate,clinical pregnancy rate,and ovarian hyperstimualtion syndrome (OHSS) rate were compared in different PDF groups with different protocols.Results:In both the GnRH antagonist protocol and GnRH agonist long protocol,the characteristics such as mean age,anti-Mullerian hormone,antral follicle count (AFC),and body mass index were comparable between groups.The number of oocytes retrieved decreased statistically significantly as the PDF and rate of matured oocytes increased.In the GnRH agonist long protocol,the rate of normally fertilized oocytes was highest in Group A (59.74 ± 31.21 vs.49.70 ± 37.95,49.67± 36.62;F=3.743,P =0.025).There were no significant differences in the rate of top-quality embryos and the clinical pregnancy rate between the groups.The clinical pregnancy rate was similar in the three groups (63.6 ％,62.5 ％,67.5 ％,respectively,x2 =0.989,P =0.911).The moderate and severe OHSS rate increased statistically significantly when the PDF increased,which was highest in group C (1.4％,3.1％,6.7％,respectively,x2 =12.014,P =0.017).In the GnRH antagonist protocol,there were no significant differences in the rate of top-quality embryos,the rate of normally fertilized oocytes,the clinical pregnancy rate,and the moderate and severe OHSS rate between the groups.The clinical pregnancy rate in Group C was higher than that in Group A (57.9％ vs.46.6％,x2 =10.850,P =0.093).Conclusions:In the GnRH antagonist protocol,PDF on the HCG day of less than 20％ may be unfavorable to the clinical pregnancy rate in PCOS.In the GnRH agonist long protocol,delaying the HCG trigger timing has no good effect on clinical pregnancy and the risk of OHSS might increase in patients with PCOS.