The dynamic regulation of histone methylation is a key mechanism for epigenetic regulation of gene expression. As histone demethylases dependent on divalent iron ions and α-ketoglutarate, the JmjC family plays an important role in plant life activities. Fifty JmjC domain-containing proteins in Arabidopsis thaliana (21) and Brassica rapa (29) are divided into seven distinct groups, with each group endowed with specific functions due to unique structural domains. Some members achieve functional specificity by recognizing specific DNA motifs or interacting with transcription factors, and others exhibit special functional modes due to mutations in their binding sites. By targeting specific genes (such as FLC, FT, WRKY family, PR genes, etc.), they regulate growth and development processes, as well as responses to multiple biotic and abiotic stresses. Focusing on inter-species divergence of Arabidopsis and Brassica, this review summarized JmjC proteins’ structural classification, substrate specificity, and mechanisms, providing a basis for dissecting plant epigenetic networks and guiding Brassica crop breeding desired bolting traits (early or late bolting), high stress resistance and so on.