The invention provides an insect preventing and controlling method, which is to feed lepidopterous insets with a mixture of siRNA for the specific gene of the lepidopterous insets and a cationic polymer, wherein the siRNA is 17 to 50 nt double-chain RNA chemically synthesized in vitro; and the cationic polymer consists of polyethyleneimine (PEI), chitosan, polylysine and gelatin. The invention also relates to a biopesticide, which uses the siRNA for the specific gene of the lepidopterous insets as an active ingredient. In the invention, the gene expression of the lepidopterous insets is inhibited by feeding the lepidopterous insets with the siRNA which contains the specific gene of the lepidopterous insets, such as an interfering insect mitochondrion composite IIIFe-S subunit gene, a cholinesterase gene, a gamma-amino butyric acid receptor gene and the like, to interfere the normal growth and development of the insects, so the effective control of the insects is realized; and pot culture experiment result shows that the biopesticide is very effective.