Ovarian follicle expansion is an important part of follicular growth and development into a dominant follicle, which is regulated by a variety of molecules and signals, including follicular cavity formation, follicular fluid accumulation and granulosa cells proliferation. Polycystic ovary syndrome (PCOS) is the most common reproductive endocrine disease in women, and patients mainly present with increased preantral follicles and ovarian polycystic lesions caused by inadequate ovarian follicle expansion. This article summarizes recent new developments concerning the physiological process of ovarian follicle expansion, related regulatory factors and mechanisms, and describes the possible factors of restricted ovarian follicle expansion in patients with PCOS, with a view to providing a theoretical basis for follicular dysplasia, ovulation disorders and other diseases caused by abnormal ovarian follicle expansion.