Currently, poly(aspartic acid) (PASP) is used with traditional toxic agents for corrosion inhibition, which greatly reduces the environmental significance of PASP as a green inhibitor. Amino acids, small-molecule compounds with amino and carboxyl groups, may react with PASP and act as chains to link PASP molecules, which might enhance the inhibition of PASP on metal corrosion. In this study, we selected glutamic acid (GLU) as a typical amino acid to explore the potential synergistic effect of the amino acid and PASP on corrosion inhibition via electrochemical experiments and molecular dynamics simulation. The corrosion inhibition of PASP was promoted by GLU with less weight loss and less pitting. The results of molecular dynamics simulation showed that GLU could bind with PASP at carboxyl groups and amino groups via donor-acceptor interactions and accelerate the diffusion of PASP to the carbon steel surface. Furthermore, the binding between PASP and the carbon steel surface can be enhanced by GLU, resulting in a dense and stable protective film. To the best of our knowledge, this is the first investigation into the mechanism of an amino acid as an enhancer to improve corrosion inhibition. This work provides a new strategy to enhance existing green inhibitors, which would significantly reduce the cost of cooling water treatment and its adverse environmental impacts.