Objective  To investigate the effects of Smad7 knock down by lentivirus on rat cardiac fibroblasts proliferation, migration, cell differentiation and collagen secretion in vitro.    Methods  The primary cardiac fibroblasts were separated from the hearts of ten SD rats and identified by immunohistochemical method. The lentivirus transfection knocked down the expresson of Smad7 in cardiac fibroblasts, Western blotting was used to detect the efficiency of Smad7 knock down by lentivirus. The proliferation of cardiac fibroblasts was quantified by real-time unlabeled cell analyzer. Cell migration was evaluted by cell wound scratch assay. Western blotting was used to detect expression of α- smooth muscle actin (α-SMA) and collagen Ⅰ（Col Ⅰ）.  Results   Myocardial fibroblasts were successfully cultured and identified by immunocytochemical methods. The multiplicity of infection(MOI)  that lentivirus transduction of myocardial fibroblasts was 100. After lentivirus transduction, 88.33% myocardial fibroblasts expressed green fluorescent protein, showed that the lentivirus could significantly reduce the protein expression of Smad7. Smad7 deficiency decreased the proliferation and migration of cardiac fibroblasts, increased the protein expression of α-SMA and decreased collagen secretion. The results indicated that Smad7 deficiency significantly down-regulated the proliferation and migration of cardiac fibroblasts, increased α-SMA protein expression and reduced ColⅠ protein expression.  Conclusion  Smad7 deficiency can significantly change the cardiac fibroblasts function , that is related to the pathological mechanism that lead to myocardial fibrosis.