In order to study the adsorption mechanism of H2O molecules on γ-TiAl (111) and α2-Ti3Al (0001) surfaces by electric field, the adsorption energy, state density, geometric structure and charge layout of H2O molecules at different adsorption positions on γ-TiAl (111) and α2-Ti3Al (0001) surfaces were analyzed by first-principles method. The results showed that the adsorption of H2O molecules was most stable at the top Ti position on the γ-TiAl (111) and α2-Ti3Al (0001) surfaces, but the electric field was more likely to promote the interaction between H2O molecules and α2-Ti3Al (0001) surfaces, that is, α2-Ti3Al was more likely to react with H2O molecules. Thus, the dense oxide film of Ti was preferentially formed, resulting in the protection of α2-Ti3Al. It is of great significance to explore the condition that γ-TiAl and α2-Ti3Al single-phase have the same dissolution rate, and to improve the surface quality of the electrochemical machining of the biphase (γ-TiAl and α2-Ti3Al) TiAl alloy.