Abstract: Hypocotyl elongation is essential for early survival and later growth and development in plants. In this investigation, we examined and screened Arabidopsis thaliana (L.) Heynh. mutant phenotypes, identifying the u2bl variant with a notable short hypocotyl. We also conducted preliminary studies on the role of U2BL in the regulation of hypocotyl development in A. thaliana. The u2bl mutant showed a short hypocotyl phenotype under different light conditions. Cytological experiments showed the shorter cell length of hypocotyls in the u2bl mutant was the reason for its short hypocotyl phenotype. The plant hormone gibberellin (GAs) is the main factor promoting hypocotyl elongation, while paclobutrazol (PAC) is an endogenous GA synthesis inhibitor. Our study showed that the u2bl mutant was not sensitive to exogenous GA treatment or PAC treatment, indicating that U2BL affected GA signal transduction. Subcellular localization results indicated that U2BL was enriched in the nucleus. Furthermore, Q-PCR assay showed that the transcription levels of PRE1, SAUR16, YUC2, YUC8, and PIF4 genes were all significantly down-regulated in the u2bl mutant, suggesting that U2BL may indirectly regulate hypocotyl elongation by regulating the above genes. Our study provided a reference for further research on the possible functions of U2BL in the growth and development of A. thaliana and other species.