Interfacial solvated electrons (${\rm e}^-_{{\rm sol}}s$) possess profound application values in physics, chemistry, and materials, thus attracting ever-growing attention. Although previous studies have unequivocally corroborated the involvement of ${\rm e}^-_{{\rm sol}}s$ in the reaction of alkali metals with water, the mechanism has not been thoroughly revealed. Here, we simulate the solvation and ionization process of a single Na or a metallic ${\rm Na}_8$ cluster at the vacuum-liquid interface by the hybrid functional-based $ab$ initio molecular dynamics (AIMD) method, especially to elucidate the interfacial electron dynamics behavior. Results show that the electron donated by Na or ${\rm Na}_8$ is partially solvated at the interface, a process driven by both the ${\rm Na}^+$ interaction with the electron and its stabilization in water, which promotes electron redistribution, delocalization, and activation. Additionally, solvation increases the $H_2O$ population near HOMO and on unoccupied orbitals, promoting $H_2O$ reorganization and electron transfer. In aqueous solutions, Na is highly ionized and generates a unique pre-solvated electron (${\rm e}^-_{{\rm pre}}$). ${\rm Na}_8$ cluster, on the other hand, is partially solvated through bottom active O-coordinating sites at the interface, polarizes internally, and produces a pre-solvated dielectron (${\rm e}^{2-}_{2 \ {\rm pre}}$), which is followed by $H_2O$ reorganization near the surface and the subsequent hydrogen evolution reaction by proton-coupled electron transfer. Surrounding $H_2O$ molecules form multiple Na-O bonds with the remaining ${\rm Na}^{2+}_8$ to compensate for ${\rm e}^{2-}_{2 \ {\rm pre}}$ loss. Our work displays the microscopic dynamics mechanism of Na and $H_2O$ reaction by AIMD simulation and provides evidence for the participation of ${\rm e}^-_{{\rm pre}}s$ in the hydrogen evolution reaction, which deepens our attention and understanding of redox reactions involving ${\rm e}^-_{{\rm sol}}s.$