MIR31HG, as the host gene of miR-31, has been suggested to involve in various cancer developments. However, little is known about the clinical significance and biological function of MIR31HG in lung adenocarcinoma. In our study, we found MIR31HG was highly expressed in lung adenocarcinoma tissues and cell lines, and associated with clinical staging, N classification, M classification and differentiated degree. Survival analysis showed MIR31HG high-expression was an independent unfavorable prognostic factor for lung adenocarcinoma patients. Loss-of-function studies suggested down-regulation of MIR31HG inhibited lung adenocarcinoma cells proliferation and blocked cell-cycle, but has no effect on cell apoptosis. There was no correlation between MIR31HG and miR-31 expression in lung adenocarcinoma tissues, down-regulation of MIR31HG had no effect on the expression of miR-31 in lung adenocarcinoma cells. In conclusion, MIR31HG high-expression is an independent unfavorable prognostic factor for lung adenocarcinoma patients, and serves an oncogenic role to modulate lung adenocarcinoma cells proliferation and cell-cycle.