Background and Objectives Systemic scleroderma (SSc) is a rare and serious connective tissue disease, an autoimmune disease, and a rare refractory disease. In this study, preventive effect of single systemic human umbilical cord mesenchymal stem cells (UC-MSCs) transfusion on SSc was preliminarily explored. Methods and Results SSc mouse model was established by daily intradermal injection of Hypochlorite (HOCl). SSc mice were treated by single transfusion of UC-MSCs at 0.625×10 5 , 2.5×10 5 and 1×10 6 respectively. At the 42nd day of intradermal injection of HOCl, the symptoms showed up by skin and alveolar wall thickening, lymphocytic infiltration, increased collagen in skin/lung, and the increased proportion of CD3 + CD4 + CD25 + FoxP3 + cells (a Treg subset) in spleen. After UC-MSCs transfusion, the degree of skin thickening, alveolar wall thickening and lymphocyte infiltration were decreased, the collagen sedimentation in skin/lung was decreased, and the proportion of CD3 + CD4 + CD25 + FoxP3 + cells was decreased. Conclusions UC-MSC can achieve a preventive effect in SSc mice by fibrosis attenuation and immunoregulation.