Background: Polycystic ovarian syndrome (PCOS) is one of the most common endocrine disease affecting the reproductive age women. Objective: To investigate the relationship between PCOS and breast, ovarian, and endometrial cancer. Search Strategy: Systematic search of PubMed, EMBASE using keywords ‘PCOS’, ‘ovarian cancer’, ‘breast cancer’ and ‘endometrial cancer’. Selection Criteria: The study providing the relative risk (RR) in the cohort study, odds risk(OR) in the case-control study, and 95%confidence interval (95%CI) were included. The single-case report and the non-English study were excluded. Data Collection and Analysis: This meta-analysis was performed by calculating RR, OR and 95%CI using random-effect models. Main Result: A total of 21 studies (8 cohort studies,13 case-control studies) involving 3831083 participants fulfilled the inclusion criteria. Based on the results of cohort studies and case-control studies, the prevalence of breast cancer among PCOS and non-PCOS women was not significant increased, the RR and OR were 0.959 (95%CI, 0.806-1.112) and 0.991 (95%CI, 0.626-1.35), respectively. Evidence from case-control studies showed that PCOS did not increase the risk of endometrial and ovarian cancer, the summary ORs of case-control studies were 1.288 (95%CI,0.763-1.814) and 1.219 (95%CI, 0.816-1.623). The risk of endometrial and ovarian cancer was significantly increased based on cohort studies, the overall RRs of cohort studies were 2.542 (95%CI, 1.755-3.328) and 1.818 (95%CI, 1.222-2.414). Conclusion: The meta-analysis demonstrate that PCOS will not increase the risk of breast cancer. Evidence from the cohort studies suggests that PCOS significantly increases the risk of endometrial and ovarian cancer, although the case-control studies did not.