Objective To study the effect and mechanism of Napsin A gene transfection into A549 cells on ephethlial-mesenchymal transition(EMT) EM>in vitro/EM> . Methods A recombinant lentiviral plasmid PLJM1-Napsin A was constructed， then transfected into A549 cell and identified. A549 cells EMT model was established by transforming growth factor beta-1（TGF-β1）treatment in vitro . The morphology change was observed under inverted microscopy successively. To observe the degree of EMT by TGF-β1 intervening A549 cells, the expression of E-cadherin and collagen type Ⅰ was detected by reverse transcription-polymerase chain reaction（RT-PCR）and Western blotting. Finally, in order to investigate the mechanism, the protein expression of focal adhesion kinase(FAK) was detected by Western blotting. Results The result of sequencing the recombinant lentiviral plasmid PLJM1Napsin A was predicted as designed. The expression of Napsin A protein in transgenic A549 cells was more than that in A549 cells and A549-PLJM1 cells（EM>P/EM>0.01）. After TGF-β1 treatment, the cells changed from epithelial-shaped into interstitial-shaped; TGF-β1 induced EMT in A549 cells, as demonstrated by significant reduction of E-cadherin mRNA and protein levels（EM>P/EM>0.01） as well as up-regulation of collagen type Ⅰ（EM>P/EM>0.01）. Transfection of Napsin A in A549 cells could not only restrain the TGF-β1-induced morphological changes, but