Based on the Volmer-Heyrovsky-Tafel mechanism, Frumkin adsorption and thermodynamic data of hydrogen (deuterium) absorption in Pd, effects of surface parameters on the hydrogen (deuterium) loading ratio into Pd in the hydrogen (deuterium) evolution reaction are discussed. There is a change of mechanism from the Volmer-Tafel route to the Volmer-Heyrovsky route when the current density rises, and there exists the maximum loading ratio at a certain current density when the symmetry factor of the Heyrovsky step is less than that of the Volmer reaction. The theoretical results fit the experimental data presented before very well; other factors that affect the loading ratio are discussed as well.