Abstract: Objective Forsythia suspensa ( Thunb. ) Vahl is a perennial woody plant with dimorphic style and self-incompatibility. Mining the candidate genes for the development of heterostyly in Forsythia suspensa is beneficial to reveal its evolution and development mechanism. Methods The whole genome resequencing of 40 long style morph and short style morph Forsythia suspensa plants were conducted, detected the SNP variation sites in the sample genome, and carry out selection sweep analysis. The candidate regions (top 5% level) were screened by the combination of population differentiation index (Fst) and nucleic acid polymorphism (θπ). Differential genes were analyzed by GO database and KEGG database functional annotation, and qRT-PCR was used to detect the expression of candidate genes at different periods of style development. Results Selective sweep analysis showed that 295 genes were selected, which mainly played an important role in plant growth and development and metabolic regulation. Some pathways related to plant hormone regulation were selected to determine the candidate genes CYP734A1 (EVM0010386), BRI1 (EVM0011829) and CYCD3 (EVM0018316) of heterostyly in Forsythia suspensa. The gene expression results indicate that the CYP734A1 gene has sustained high expression during the critical period of development in F. suspensa 's S-morph flowers. The BRI1 and CYCD3 genes in the S-morph were significantly higher than those in the L-morph during the crown exposure period. Conclusion The candidate genes excavated in this study were differentially expressed in the L-morph and S-morph style of F. suspensa. Brassinosteroids was involved in the development of the heterostyly in F. suspensa, which was one of the reasons for heterostyly of F. suspensa. This study laid a foundation for the evolution and genetic development regulation mechanism of heterostyly in F. suspensa.