Abstract: The Microbacterium imperiale of Mebl-012 was selected as the original strain,and the composite mutagenesis of Atmospheric and room temperature plasma( ARTP)-UV were used to treat it,in order to obtain a strain of high maltotriose-forming enzyme activity. The bacterial suspension under different ARTP mortality rates were mixed,and then were coated filter plate,followed by fermentation rescreening. Eventually a high-yielding strain Microbacterium imperiale of Metp-57 was screened out,production of this strain reached 241.32 U/m L,which was increased by 107.26% comparing with the original strain Mebl-012( yield of 116.43 U/m L). Choosing high-producing strain Metp-57 as starting strain,UV mutagenesis was conducted,maltotriose-forming enzyme activity of the mutant Metu-24 reached 408.41 U/m L,which was increased by 250.77% compared with the original strain Mebl-012,and its genetic stability was good.Compared with the original strain Mebl-012,the growth rate of metant strains Metu-24 was significantly improved.Hydrolyzing starch by this enzyme,maltotriose was predominant component,maltotriose content reached 46.7% in maltotriose syrup.