Abstract: Objective To investigate the expression of miRNA-let-7a in polycystic ovarian syndrome (PCOS) mouse model and COV434 cells and its effects on PI3K, so as to provide new ideas and targets for the treatment of infertility characterized in PCOS. Methods The levels of estradiol (E2), luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone (T) in PCOS mice were detected by radioimmunoassay. Then the mice were sacrificed through cervical dislocation, and bilateral ovaries were collected and weighted. The number of eggs in each group was counted under a microscope. The expression of miRNA-let-7a in the ovarian tissue of PCOS mice was detected by qRT-PCR. After miRNA-let-7a overexpression and silencing, the expression of PI3K in human ovarian granular cell line COV434 was detected by qRT-PCR and Western blot. Results The level of miRNA-let-7a in ovarian tissue of PCOS mice significantly increased. After overexpression of miRNA-let-7a, the levels of PI3K mRNA and protein in COV434 cells significantly increased. When miRNA-let-7a was inhibited, the levels of PI3K mRNA and protein in COV434 cells significantly decreased. Conclusions The expression of miRNA-let-7a in ovarian tissue of PCOS mice is significantly enhanced, which may regulate the expression of PI3K and then regulate the growth and development of ovarian granular cells.