In this study, Brassica napus L. ‘Xianyou 15’ was used as the test material. By bioinformatics analysis, the CRISPR/Cas9 knockout sites were designed for the BnaSDG8–A and BnaSDG8–C conserved sequence regions on the first exon of BnaSDG8. The CRISPR/Cas9 knockout vector pCAMBIA1300–sgRNA/Cas9–BnaSDG8 was successfully constructed; 19 transformed plants were obtained through genetic transformation， 4 plants were successfully edited in 19 transformed plants. In addition, through the statistical analysis of flowering time of directional knockout plants, the BnaSDG8 is involved in the regulation of flowering time of Brassica napus L.