The invention relates to a preparation method for a WO3-LDH (Layered double hydroxide) composite film for photoelectrocatalytic water decomposition. The preparation method comprises the following steps of: dissolving ammonium tungstate and concentrated hydrochloric acid into water to obtain a solution, stirring the solution and transferring the solution into a lining of a high-pressure reaction kettle, and inserting pre-treated conductive glass FTO; then, carrying out solvothermal reaction, naturally cooling the room temperature, washing and drying the conductive glass, and carrying out thermal treatment for at least one hour at a temperature being 450-500 DEG C under air atmosphere, thereby obtaining a WO3 film; and the WO3 film as a working electrode, taking an aqueous solution of nickel nitrate and iron nitrate as electrolyte, depositing NiFe-LDH on the surface of the WO3 film under constant current, finally obtaining the WO3-LDH composite film. The product prepared by the preparation method has high photoelectric conversion efficient, low photooxidation water take-off potential, and has a wide application prospect in the fields such as photocatalytic degradation, photoelectrocatalytic water decomposition, artificial photosynthesis and photo-assisted energy storage batteries.