A simple and rapid screening method for amino acid dehydrogenase (e.g., leucine dehydrogenase, LDH) has been developed. It relies on a competitive relationship between a non-fluorescent Cu(II)-calcein complex and amino acid (e.g., l-2-aminobutyric acid, l-ABA). When ABA was introduced to a Cu(II)-calcein solution, it bound with the Cu(II) ions and this released calcein from the complex, which was detected as strong fluorescence. The principle of this high-throughput screening method was validated by screening an LDH mutant library. Compared with other methods, this method provided much quicker l-ABA detection and screening for leucine dehydrogenase mutations.