Surface states on TiO2 photoanodes are usually considered to hinder the photoelectrochemical (PEC) water splitting via slowing charge transport and increasing charge recombination. Here, we found that electro-chemically doping the TiO2 nanotubes electrode at a potential negative than its flat band potential will induce the insertion of protons into the lattice of TiO2 via a reaction of (TiO2)-O-IV + e(-) + H+ = (TiO)-O-III(OH) to form the surface states. Photoelectrochemical/electrochemical tests confirmed that the surface states (Ti-OH) are an important photogenerated electron transfer route, significantly increasing charge separation efficiency, and hence contributing to the PEC performance enhancement. In addition, we have made a self-consistent explanation on Mott-Schottky (M-S) plots of the TiO2 nanotubes electrode, demonstrating that the M-S measurement is able to accurately describe electron filling and extraction into/from surface states and not suitable to determine the flat band potential of nanostructured TiO2 electrodes. Our results offer a new insight to understanding the role of surface states during the charge separation, transfer, and injection processes of the water splitting reaction.