Smooth muscle cell (SMC) differentiation is an important process during vasculogenesis and angiogenesis. However, molecular mechanisms controlling VSMC differentiation are not completely understood. Here, we identified a novel role of Mesenchyme homeobox 1 (Meox1) in SMC differentiation. Transforming growth factor-β (TGF-β) induces Meox1 expression in the initial phase of SMC differentiation of pluripotent C3H10T1/2 cells. Knockdown of Meox1 by specific shRNA suppresses TGF-β-induced expression of SMC early markers in both in vivo and in vitro. Meox1 overexpression increased these markers expression. Mechanistically, Meox1 promoted high level Smad3 nuclear retention during the early stage of TGF- TGF-β stimulation, which was achieved by inhibiting phosphorylated Smad2/3 phosphatase, Mg2+/Mn2+ dependent 1A (PPM1A) expression through increasing its degradation rate, leading to a sustained level of Smad3 phosphorylation and thus allows Smad3 to precisely regulate SMC marker gene transcription. SMC is also the main component of the fibrous cap which is participated in plaque stabilization during atherosclerosis progression. Therefore, elucidating mechanisms controlling VSMC function during the development of atherosclerosis is critical for the understanding of disease formation. Here, by using Smad2-SMC knockout LDLR-/- mouse model, we found SMC Smad2 deficiency caused a larger atherosclerotic plaque area in the whole aorta but not the aortic root and led to an unstable plaque formation due to the increased MMP2/9 expression and activity. Also, SMCs Smad2 was involved in impeding SMC to macrophage-like cells transdifferentiation. SMC has multiple functions during abdominal aortic aneurysm (AAA) formation. However, the role of SMC Smad2 during AAA development is still unknown. By using a SMC-specific Smad2 disruption animal model undergone elastase induced AAA formation, we found that Smad2 deficiency in SMC increased vessel diameter and AAA incidence as well as increased extracellular matrix degradation and SMCs loss. Additionally, we observed the increased aorta media layer matrix metalloproteinases MMP-2 and MMP-9 expression and a thinner adventitia layer with less fibroblasts proliferation when Smad2 was deleted in SMC. Moreover, we found SMC-derived factors may partially lead to decreased fibroblasts proliferation in Smad2 sm-/- mice AAA model. RNA-seq screening indicated that thrombosponding1 (TSP1) might be a potential secreted factor during this process.