The invention relates to a preparation method of a NiO photocathode based on self-assembly of zinc porphyrin and cobalt dimethylglyoximate. The preparation method aims at solving the problems that existing photocathode preparation methods are complex, photocathodes are low in water decomposition efficiency, and the hydrogen production speed is low. The preparation method comprises the steps of 1, preparing a precursor solution of a NiO compact layer; 2, preparing the NiO compact layer; 3, preparing slurry; 4, preparing a NiO blank piece; 5, preparing a NiO photocathode sensitized by ZnTHPP; 6, preparing a NiO photocathode sensitized by ZnTHPP-bpy; 7, preparing a NiO photocathode sensitized by self-assembly of ZnTHPP-bpy-Codmg; 8, preparing a NiO photocathode sensitized by ZnTPPC; 9, preparing a NiO photocathode sensitized by ZnTPPC-bpy; and 10, conducting soaking and cleaning. By means of the preparation method, the NiO photocathode based on self-assembly of the zinc porphyrin and the cobalt dimethylglyoximate can be obtained.