Construction of photocatalytic Z-scheme overall water splitting (OWS) using iodate/iodide (IO3-/I-) redox mediator commonly confronts challenges of competing reactions and slow reaction kinetics. Here we address the aforementioned key issues using surface modification strategy. Visible-light-responsive tantalum nitride (Ta3N5) and tantalum oxynitride (TaON) are employed as the O2- and H-2-evolving photocatalysts, respectively. It is found that the inhibition of competing reactions and promotion of half reactions are indispensable for the successful construction of Z-scheme OWS system, both of which are directly related to the surface property of the photocatalysts. Specifically, the magnesia modification on the surface of Ta3N5 is demonstrated to be effective not only in suppressing the adsorption and oxidation of ions (a competing reaction of water oxidation), but also in promoting the charge separation and O-2-evolving rate via meliorative dispersion of loaded iridium co-catalyst. And surface coating of Cr2O3 nanolayer on the platinum cocatalyst loaded on the TaON-based photo catalyst is available to inhibit the reduction of IO3- ions (a competing reaction of proton reduction) and improve the H-2-evolving rate. Only after both inhibition of competing reactions and promotion of H-2- and O-2-evolving reactions under the assistance of surface modification, the visible-light-driven Z-scheme OWS system can be achieved successfully with the mixture of IO3- and I- ions as a redox mediator. This work sheds light on the availability and importance of surface modification strategy in fabricating Z-scheme OWS system.