Abstract Light is a necessary environmental factor for plant growth and development. HY5 (ELONGATED HYPOCOTYL 5), regarded as a crucial transcriptional factor for regulating hypocotyl, was clearly in Arabidopsis thaliana, but its regulatory mechanism in non-heading Chinese cabbage (NHCC) has not been understood. In this study, we used BcHY5 as a bait to screen two BcB-box (BBX) proteins, BcBBX21, and BcBBX25, and applied multiple genetic and biochemical approaches to identify the roles of BcHY5 and two B-BBX proteins. Overexpression of the two BBX proteins in Arabidopsis was used to investigate their functions. BcHY5 can bind the G-box of pro BcMYBH and inhibit BcMYBH transcription. BcHY5 can form a complex with BcBBX21, which promotes BcHY5’s regulation of BcMYBH and increases the transcription inhibition of BcMYBH, resulting in the shorter hypocotyl. BcBBX25 can interact with BcHY5 to release the inhibition of BcMYBH by BcHY5 , allowing the hypocotyl to return to normal. The C-terminal exchange confirmed that the interaction between BcBBXs and BcHY5 was determined by the two B-box domains of the N-terminal and the opposite effect of BcBBXs on BcHY5 was caused by C-terminal difference. The current results demonstrate the fine-tuned regulation of hypocotyl length via transcriptional regulation of BcMYBH by BcHY5-associated proteins and provide insights into Cruciferous hypocotyl length.