MicroRNAs (miRNAs) play a significant role in tumor progression, and regulating miRNA expression with small molecules may offer a new approach to cancer therapy. Among them, miRNA-20b has been found to be dysregulated in several cancers, including nonsmall cell lung cancer (NSCLC). Herein, an in silico high-throughput computer screen was conducted to identify small molecules that downregulate miR-20b using the three-dimensional structure of the Dicer binding site on pre-miR-20b. Among 1058 small molecule compounds, Methotrexate (MTX), was discovered to be a potential miR-20b-specific inhibitor, which has been found to suppress miR-20b by specifically blocking Dicer processing in p53 wild-type A549 NSCLC cells but not in H1299 cells with p53 depletion. MTX effectively inhibited the proliferation, survival, migration, and invasion of A549 cells in a dose-dependent manner. Furthermore, the treatment of MTX up-regulated the expression of miR-20b target genes PTEN, STAT3, and HIF1α. Notably, MTX also significantly inhibited tumor growth in a mouse xenograft tumor model of NSCLC, with no observed tissue toxicity. Our findings indicate that MTX may have a novel role as an established drug in p53 wild-type NSCLC tumor therapy by down-regulating miR-20b expression. These findings are expected to provide preclinical evidence for miR-20b-targeting NSCLC therapeutic strategies.