Disclosed is a method for introducing an exogenous DNA by overcoming the restriction modification barrier of the target bacterium. The method includes the steps of 1) co-expressing all DNA-methyltransferase-encoding genes in the genome of the target bacterium in E. coli in which the restriction modification system thereof has been deleted to obtain a recombinant bacterium A, 2) introducing an exogenous DNA molecule into the recombinant bacterium A for in vivo modification so as to obtain a methylation-modified exogenous DNA molecule, and 3) introducing the methylation-modified exogenous DNA molecule into the target bacterium.