Because of the stagnation of hypocotyl elongation of Lepidium apetalum Willd, at low temperature, it can be used as a good material to study the effect of temperature on hypocotyl elongation. In order to investigate whether the transcription factor HY5 plays a key role of L. apetalum hypocotyl elongation under the low temperature, we cloned and sequenced the LaHY5 sequence from the seed transcriptome of L. apetalum Willd. The relationship between the gene expression and low temperature induction and germination stage were studied by realtime quantitative PCR, the effect of the gene expression on hypocotyl elongation under low temperature was analyzed by transformation of Arabidopsis. The results showed that： (1) the LaHY5 cDNA sequence contains 447 bp of complete reading frame sequence, and its coding product is a peptide chain composed of 149 amino acids rich in serine, which contains a typical BRLZ domain. The relative molecular weight is 16.830 kD, the molecular formula is C₆₉₂H₁₁₅₆N₂₂₈O₂₄₆S₇, and the theoretical isoelectric point is 8.73, which is highly consistent with the homologous sequence of L. apetalum Willd. (2) The gene was rapidly upregulated by low temperature induction in the seeds or seedlings during the germination of L. apetalum Willd. (3) Under normal or low temperature conditions, the hypocotyls of transgenic Arabidopsis seeds grew faster than those of wildtype Arabidopsis. These results indicate that LaHY5 transcription factor plays an important role in seed germination of L. apetalum Willd, and seedling tolerance to low temperature stress. However, it also proved that it was not the cause of low temperature stagnation for hypocotyls elongation of L. apetalum Willd.