Alpha-amylase inhibitors are widely used by the pharmaceutical and agricultural industries, such as the treatment of diabetes and obesity and insect controller. Here, we developed a colorimetric method to screen for ${alpha}$-amylase inhibitor producing strains or mutants with higher ${alpha}$-amylase inhibitor productivity. This method relies on absorbance changes at 402 nm that are due to the inhibition of ${alpha}$-amylase catalyzed hydrolysis of 2-Chloro-4-nitrophenyl-4-O-${eta}$-D-galactopyranosyl-maltoside by ${alpha}$-amylase inhibitors. The assay can be performed on a microtiter plate, making it simple and convenient. Using this method, ${alpha}$-amylase inhibitor producing strains and mutants with higher ${alpha}$-amylase inhibitor productivity can be rapidly screened. One strain, ZJB-08196, with the highest ${alpha}$-amylase inhibition was isolated and identified as Actinoplanes utahensis, and one mutant with higher acarbose production was obtained by screening 3,000 variants using this method.