Abstract: Cyclin-dependent kinases (CDKs) are not only responsible for cell cycle regulation in cells, but also play an important role as regulators in cell transcription. CDK9, a member of the CDKs family, plays an important role in cellular transcriptional regulation. CDK9 associates mainly with Cyclin T1 and forms the positive transcription elongation factor b (P-TEFb), which regulates the transcription elongation by phosphorylating the carbon-terminal domain (CTD) of RNA polymerase Ⅱ (Pol II). CDK9 inhibitors inhibit CDK9-mediated transcriptional extension by competitive binding. This review briefly introduces the functions of CDK9 and its mechanisms of action in tumor, and summarizes the progress in the study of CDK9 inhibitors.