Plant trichomes formed by specialized epidermal cells play a role in protecting plants from biotic and abiotic stresses and can also influence the economic and ornamental value of plant products. Therefore, further studies on the molecular mechanisms of plant trichome growth and development are important for understanding trichome formation and agricultural production. SET Domain Group 26 (SDG26) is a histone lysine methyltransferase. Currently, the molecular mechanism by which SDG26 regulates the growth and development of Arabidopsis leaf trichomes is still unclear. We found that the mutant of Arabidopsis () possessed more trichomes on its rosette leaves compared to the wild type (Col-0), and the trichome density per unit area of is significantly higher than that of Col-0. The content of cytokinins and jasmonic acid was higher in than in Col-0, while the content of salicylic acid was lower in than in Col-0, which is conducive to trichome growth. By measuring the expression levels of trichome-related genes, we found that the expression of genes that positively regulate trichome growth and development were up-regulated, while the negatively regulated genes were down-regulated in . Through chromatin immunoprecipitation sequencing (ChIP-seq) analysis, we found that SDG26 can directly regulate the expression of genes related to trichome growth and development such as , , , , , , , , , , , and by increasing the accumulation of H3K27me3 on these genes, which further affects the growth and development of trichomes. This study reveals the mechanism by which SDG26 affects the growth and development of trichomes through histone methylation. The current study provides a theoretical basis for studying the molecular mechanism of histone methylation in regulating leaf trichome growth and development and perhaps guiding the development of new crop varieties.