More and more studies are investigating the influence of the expression of MicroRNA-21 (miRNA-21) on prognosis and clinical significance in patients with lung cancer, but the results are contradictory and uncertain. A meta-analysis was conducted with controversial data to accurately assess the issue.A detailed search of relevant research in Wanfang, Chinese Biomedical Literature Database, Chinese National Knowledge Infrastructure (CNKI), the Chongqing VIP Chinese Science and Technology Periodical Database, PubMed, Embase, Web of Science and other databases. Two reviewers independently conducted data extraction and literature quality evaluation. Odd ratio and its 95% confidence intervals were used to evaluate the relationship between miRNA-21 and clinicopathological characteristics of lung cancer patients. Hazard ratios and its 95% confidence intervals To assess the prognostic effect of miRNA-21 on overall survival and disease-free survival. Meta analysis was performed using RevMan 5.3 and Stata 14.0 software.This study will provide a high-quality evidence-based medical evidence of the correlations between miRNA-21 expression and overall survival, disease-free survival and clinicopathological features.The study will provide updated evidence to evaluate whether the expression of miRNA-21 is in association with poor prognosis in patients with lung cancer.The private information from individuals will not publish. This systematic review also will not involve endangering participant rights. Ethical approval is not available. The results may be published in a peer- reviewed journal or disseminated in relevant conferences.