<p>Leaf morphogenesis is governed by the precise transcriptional regulation of developmental genes, which coordinates both organ boundary formation and morphological patterning. However, the molecular mechanisms involved in regulating leaf morphological development in poplar remain poorly understood. In this study, three lines of poplars (<i>Populus simonii</i> × <i>P. nigra</i>) with significant differences in leaf shape and phenotype were selected as experimental materials, and leaf morphological traits of different lines were determined. In addition, RNA-seq transcriptome sequencing technology was used to screen differentially expressed genes (DEGs) in order to reveal the molecular regulatory mechanism of poplar leaf morphogenesis. Results showed significant leaf size variations among different lines: B6-44 and B7-71 exhibited smaller leaf area and lower aspect ratios than B7-18. Cellular analysis revealed that B6-44 and B7-71 had smaller adaxial epidermal cells but higher cell density and elongated shapes compared to B7-18, while the abaxial epidermis showed opposite trends. Additionally, B7-18 demonstrated more developed xylem, palisade, and spongy tissues in main veins than the other two lines. Through comprehensive transcriptomic analysis, we identified <i>PsnERF1</i> as a key differentially expressed transcription factor in poplar leaf development. To investigate its functional role, we generated ERF1-overexpressing transgenic lines in 84&#xa0;K poplar (<i>Populus alba</i> × <i>P. glandulosa</i>). Compared with wild-type plants, overexpression of <i>PsnERF1</i> significantly inhibited leaf development. This study significantly advances our understanding of the molecular mechanisms governing poplar leaf morphogenesis and development, while providing a novel research approach for the development of new poplar varieties.</p>