Promoting the hole extraction from the photocathode semiconductor is crucial to not only enhance the charge separation and suppress the charge recombination but also to protect the oxidation of the photocathode semiconductor by the photogenerated holes. Here, we use a very thin MoO3 film as a hole buffer layer between conductive substrate fluorine-doped tin oxide and the p-type semiconductor CuBi2O4. Through comprehensive photoelectrochemical characterizations, we find that the insertion of a hole buffer layer MoO3 not only accelerates the hole traction from the CuBi2O4 photocathode but also blocks the backward transfer of photogenerated electrons. This optimized charge transfer behavior contributes to the improved photoelectrochemical performance. Based on our results, some interesting designs on CuBi2O4 photocathode are given at the end that will be potentially working as effective photocathodes.