Polycystic ovary syndrome (PCOS) is a group of endocrine disorder syndrome with sporadic menstruation or amenorrhea, hirsutism, obesity, infertility and bilateral ovarian enlargement with cystic changes, patients may have the above typical symptoms or only some of them, but infertility due to ovulation disorder is the main clinical manifestation of PCOS. Clomiphene is a non-steroidal compound with dual anti-estrogenic and weak estrogenic activity. Low doses promote gonadotropin secretion from the anterior pituitary, which induces ovulation. Clomiphene induces ovulation at a rate of 70C80%, and about l5-20% of PCOS patients with clomiphene are persistently anovulatory and have a low pregnancy rate of 30C40%. The anti-estrogenic effect of clomiphene on the endometrium and cervix decreases the pregnancy rate. Letrozole is a third-generation aromatase inhibitor, which blocks estrogen synthesis by inhibiting aromatase activity, decreases estrogen levels in the body, induces increased secretion of endogenous prohormones, and stimulates follicular development. Its effect on endometrium and cervical mucus is small and suitable for embryo implantation, growth, and development, and its half-life is short and does not occupy estrogen receptors. Letrozole is an effective ovulation-promoting drug that does not inhibit endometrial It does not inhibit the development of endometrium and induces the occurrence of a single dominant follicle, which can prevent the occurrence of multiple pregnancies and reduce the risk of ovarian hyperstimulation syndrome.