To understand the effect of brassinolide on quality and dim light reaction of Brassica napus L., 2 ir? radiated mutants (XY881 and XY883) of B. napus cv Xiangyou 15 were investigated to study the effects of epibrassi? nolide (2,4-BL) on seed germination, vegetative growth, oil production and related components by pot experiment and field trials from 2014 to 2016. Results showed that 2,4-BL had the same effects on both lines. 2,4-BL had dose effect on seed germination. Low concentration promoted seed germination. Low 2,4-BL promoted elongation of main stem, increased photosynthetic efficiency, and promoted fruiting without affecting seed size and seed oil. Using 2,4- BL after flowering delayed seeds ripen and led to lower oil content. Regulation of 2,4-BL decreased in the case of long-term continuous use. Gene expression data showed that 2,4-BL induced (BnaPIF4, BnaBZR1 and BnaBES) gene expression of the key transcription factors in brassinosteroid signalling pathway and the light signalling path? way. It suggested that 2,4-BL activated downstream regulatory gene network to promote vegetative growth and si? lique growth by promoting photosynthesis. Thus 2,4-BL could be used as growth regulator in rapeseed production, but be applied at low concentration or long intervals, with the best result before the full flowering stage.