Effective delivery of therapeutic biomolecules across biomembranes is a challenging topic. A cationic cyclopeptide named TD-34 (ACSSKKSKHCG) was reported to improve insulin delivery across biomembranes effectively. Based on our previous work, we investigated the mechanism of TD-34 for enhancing insulin across Caco-2 cell monolayers. Transport studies of insulin, TD-34 and insulin accompanied with TD-34 were performed respectively using Caco-2 cell monolayers at different conditions. Transepithelial electrical resistance (TEER) value was monitored for 24 h immediately after the beginning of transport experiments. Moreover, the tight junction protein (Claudin-1) was localized by confocal immunofluorescence microscopy. Results showed the transport of insulin alone across biomembranes was attributable to multiple routes including passive diffusion. When TD-34 accompanied with or without insulin was treated on Caco-2 cell monolayers, TEER values decreased reversibly, and it was correlated with the reappearance of tight junction proteins by immunostaining assay. It was concluded that the cationic cyclopeptide (TD-34) had the potential to enhance paracellular delivery of insulin across Caco-2 cell monolayers by loosening tight junction reversibly.