Down syndrome (DS) is caused by an extra copy of chromosome 21 (Hsa21). Children with DS have an increased frequency of respiratory tract infections, impaired alveolar and vascular development, and pulmonary hypertension. How trisomy 21 causes lung diseases remains poorly understood. In this study, we use the Dp16 mouse model, which contains a segmental chromosomal duplication of the entire Hsa21 syntenic region on mouse chromosome 16, to explore the gene dosage effects on DS-related lung diseases. The Dp16 mice present impaired alveolar development and inflammatory-like pathological changes. Single-cell transcriptome (scRNA-seq) analysis highlights increased APP-related interactions among male Dp16 lung cells. Specifically, altered antigen processing and presentation with increased MHC-II signaling are found in Dp16 immune cells. Reduced angiogenesis and altered inflammatory responses of Dp16 endothelial cells are also suggested. Moreover, scRNA-seq indicates hyperplasia of Dp16 vascular smooth muscle cells, which is validated by tissue immunofluorescence assessment. Transthoracic echocardiography further shows the existence of pulmonary hypertension in young Dp16 mice. Independent scRNA-seq analysis of the female lung cells recapitulates the majority of key findings identified in male mice, confirming the reproducibility of the results. Collectively, our results provide important clues for the further development of therapeutic approaches for DS-related lung diseases.