Dtype cyclin regulates the G1/S transition of the cell cycle and plays an important role in the growth and development of plants. Transgenic poplar PtoCYCD2;1 (OEPtoCYCD2;1) plants showed obvious phenotypic changes, with reduced plant height, thinner stems and curled leaves. Using transgenic poplar OEPtoCYCD2;1 as the research material, the study analyzed the function of PtoCYCD2;1 in plant growth and development by linking plant phenotypes to transcriptomics sequencing and physiological index changes. This study provides a theoretical basis for the functional study of Dtype cyclins in woody plants. Studies have shown that: (1) a total of 1 269 differentially expressed genes were identified in OEPtoCYCD2;1, of which 700 were upregulated and 569 were downregulated. Through analysis, it was found that 26 genes belonging to AP2/ERF transcription factors were upregulated; 8 downregulated differentially expressed genes were enriched in the xylem synthesis pathway; a total of 27 downregulated differentially expressed genes were enriched in the carbon metabolism pathway. In the carbon metabolism pathway, 8 genes were enriched in the Calvin cycle pathway. (2) The results of qRTPCR experiments showed that the qRTPCR results of 9 differentially expressed genes were consistent with the trend of expression levels determined by RNAseq, indicating the reliability of the RNAseq results. (3) Physiological index analysis showed that compared with wild type (WT), the total chlorophyll content of young and mature leaves of transgenic poplar OEPtoCYCD2;1 increased by 57.36% and 78.22%, respectively; and the soluble sugar content of mature leaves decreased by 12.72%; and the lignin content in young and mature leaves decreased by 4.48% and 8.03%, respectively and the lignin content in young and mature stems decreased by 20.03% and 31.63%, respectively. This study indicated that transgenic poplar OEPtoCYCD2;1 affects the expression of related genes in the carbon metabolism and lignin synthesis process of poplar, thereby reducing the contents of corresponding metabolites in transgenic plants, eventually leading to changes in plant phenotype and overall biomass reduction.