In order to investigate the inhibiting effect of RNAi on the proliferation of Bombyx mori nucleopolyhedrovirus (BmNPV), BmN cells were transfected with transgenic vector pigA3-LEF-Neo containing lef-1 dsRNA expression cassette, to produce stable transformation cell line screened with G418(750～800 mg/L). Virus infection test showed that (1) the infection ratio of the stable transformation cells was lower than that of normal cells by 53%, (2) the quantity of polyhedron from transformation cells were 2/3 of that of normal cells and (3) the number of liberation virus from cell culture′s supernatant decreased more than 90%, suggesting that the virus′ proliferation in the transformation cells was depressed significantly. The result underlying the transcriptional level of the lef-1 gene in the transformation cells was only 2/5～3/5 of that in the normal cells suggested that the transcribed lef-1 dsRNA in transformation cells inhibited the expression of the lef-1 gene. Inverse PCR was used to locate the site of exogenous DNA fragment insertion in genome, of which the result showed that in the transformation cells, exogenous DNA could be inserted into the cell genome randomly or at TTAA target sequence specifically for piggyBac element transposition.