The invention relates to a preparation method of a NiCoAl-LDH-modified ferric oxide composite photoanode material. The preparation method comprises the specific steps that 1), Ni(NO3)2, Co(NO3)2 and Al(NO3)3 are dissolved into deionized water and prepared into a mixed salt solution, and the total molar concentration of the mixed salt solution ranges from 0.010 mol/L to 0.200 mol/L; 2), urea and NH4F are added to the mixed salt solution, wherein the molar concentration of the urea ranges from 0.025 mol/L to 0.500 mol/L, and the concentration of NH4F ranges from 0.010 mol/L to 0.200 mol/L; 3), the mixed solution obtained in second step is transferred into a hydrothermal kettle, and a Fe2O3 electrode plate is put into the hydrothermal kettle, a constant-temperature reaction is conducted in a drying oven at the temperature of 80-150 DEG C for 1-6 hours, taking-out is conducted after the reaction is completed, washing and drying are conducted, and then the NiCoAl-LDH-modified ferric oxide composite photoanode material is obtained. According to the preparation method, FTO loaded with Fe2O3 serves as a substrate, the in-situ growth of the NiCoAl-LDH activity protection layer is conducted through a hydrothermal method, the conductivity of a catalyst is improved, and the separation and migration of a photon-generated carrier are promoted; meanwhile active sites are provided for oxidation of water or biomass, and the reaction overpotential is lowered effectively; and migration from H+ to the solution can be promoted as well, the Fe2O3 hydrolysis is inhibited, and the stability of the NiCoAl-LDH-modified ferric oxide composite photoanode material is improved.