Abstract: The traits of flower types are significant ornamental features of Camellia japonica. In order to reveal the structural characteristics of the SEP genes in C. japonica and its relationship with the double-petal flower type, homologous cloning combined with RACE method was used to clone the SEP genes, bioinformatics analysis was performed, and real time fluorescence quantitative PCR was used to detect the expression characteristics of the SEP genes in different flower types of C. japonica. Results showed that three SEP genes were cloned from C. japonica ‘Yanzhimudan’: CjSEP1a（1235 bp）, CjSEP3（967 bp）, and CjSEP4a（1725 bp）, with their respective GenBank accession numbers OP172861, OP172863 and OP172864. These genes encoded proteins consisting of 249, 185 and 243 amino acids, respectively. All three SEP genes encoded unstable, hydrophilic proteins localized in the nucleus, featuring conserved domains of MADS-box and K-box, and exhibited high similarity to SEP proteins from C. sinensis. The qPCR analysis indicated that the expression levels of these CjSEP genes in the fully double-petal cultivar ‘Yanzhimudan’ were significantly upregulated compared to those in other cultivars, followed by the semi-double-petal cultivar and the single-petal cultivar. Interestingly, the expression levels of CjSEPs in three other specific double-petal cultivars were generally lower. Therefore, as the prominence of double-petal phenotype increases, the expression of SEP genes also rises. A strong relationship exists betwwn CjSEP genes and the characteristic double-petal trait in C. japonica, with CjSEP genes exhibiting synergistic effects in their expression.