The invention discloses a preparation method of a tungsten trioxide composite photoelectrode. The preparation method comprises the following steps: firstly, generating a tungsten trioxide film on a piece of conductive glass so as to obtain a substrate; secondly, performing hydrothermal and annealing treatment on the substrate so as to obtain a tungsten trioxide nanosheet photoelectrode; thirdly, coating the tungsten trioxide nanosheet photoelectrode with graphene oxide by using a spin-coating method, and annealing so as to obtain a reduced graphene modified tungsten trioxide photoelectrode; and in a three-electrode system, by taking the reduced graphene modified tungsten trioxide photoelectrode as a working electrode and taking a solution containing ferric trichloride, nickel chloride, sodium fluoride, potassium chloride and hydrogen peroxide as electrolyte, performing cyclic loop sweeping by using a cyclic voltammetry method, and washing and drying the working electrode, thereby obtaining the tungsten trioxide composite photoelectrode. As reduced graphene and nickel doped hydroxyl iron oxide are adopted to modify the tungsten trioxide photoelectrode, the conductivity of the photoelectrode and the transfer efficiency of a photon-generated carrier are improved, an oxygen separation reaction of an electrode interface is promoted, and the efficiency of the photoelectrode in decomposition of water through photoelectrocatalysis is improved.