Cosolvency is an intriguing thermodynamic phenomenon. However, the lack of theoretical research restricts its development and its further applications. In this work, l-alanine, l-phenylalanine, and L-tryptophan were selected as model substances to investigate the mechanism of cosolvency at the molecular level. First, the dissolution behaviors of three amino acids were characterized to determine the solvent ratios at the occurrence of the cosolvency phenomenon. Furthermore, amino acid molecules undergo a shift in molecular conformation, which leads to changes in inter/intramolecular interactions. A molecular dynamics simulation method was proposed to calculate the trends of inter/intramolecular interactions, demonstrating that the maximum point of the inter/intramolecular interaction ratio exactly corresponds to the occurrence of the cosolvency. The cosolvency phenomenon of l-proline and l-threonine was predicted successfully based on this simulation method. These results are likely to provide in-depth comprehension and guidance for predicting the cosolvency phenomenon of amino-acid-like substances.