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In this paper, the transformation water activity of L-Phenylalanine (L-Phe) in a methanol-water solution at a different temperature was measured by the ternary diagram. The influence of water activity on the transformation temperature and on the transformation rate was investigated. The solubility of the metastable form of different water activities was estimated based on the thermodynamic equilibrium and the results showed that the anhydrous L-Phe can be synthesized at room temperature by decreasing water activity, the transformation rate from monohydrate to anhydrate was enhanced with the increase of water activity. The predicted solubility value is in good agreement with the experimentally obtained. Meanwhile, tailor-made additives were applied into the system as to investigate the transformation behavior of L-Phe. Their mechanism was proposed based on the Langmuir model fitting. The result manifested that the addition of L-Trp/L-His amino acid in the L-Phe solution decelerated the transformation rate. When the concentration of L-Trp amino acid reached 0.7 × 10−4 mol/mL, the transformation behavior of L-Phe can be fully impeded, and the transformation behavior can be partially inhibited with the addition of L-Trp at 313.15 K or with the addition of L-His at 283.15 K/313.15 K.