The development of low-cost and high-efficiency photocatalysts is an important way to realize photocatalytic hydrogen production. In this work, polyvinylpyrrolidone (PVP) is used as a morphological modifier to prepare a three-dimensional (3D) sea-urchin-structure NiCo₂O₄. Mn_0.05Cd_0.95S and NiCo₂O₄ spontaneously assemble into a spatial structure of the NiCo₂O₄–Mn_0.05Cd_0.95S p–n heterojunction under the action of the Coulomb force. The 3D sea-urchin-structure NiCo₂O₄ provides support for the Mn_0.05Cd_0.95S particles in the space, which improves the dispersion of Mn_0.05Cd_0.95S and exposes more reaction sites. Furthermore, the NiCo₂O₄–Mn_0.05Cd_0.95S composite catalyst with a spatial structure has good light absorption capacity. Mn_0.05Cd_0.95S is tightly bound on the surface of NiCo₂O₄, forming a built-in electric field at the contact interface to induce the directional migration of photogenerated carriers and effectively inhibit the recombination of electrons and holes. Photoelectrochemical and fluorescence tests show that the appropriate conduction band and valence band positions of NiCo₂O₄ and Mn_0.05Cd_0.95S, and the built-in electric field of the p–n heterojunction are more conducive to thermodynamic charge transfer. Because the composite catalyst has a large number of reaction sites, and shows faster charge transfer speed and effective inhibition of the recombination of electrons and holes, the hydrogen production rate of 3%NiCo₂O₄–Mn_0.05Cd_0.95S is as high as 17 008 μmol g⁻¹ h⁻¹, which is 4.45 times that of pure MCS. This work will provide new ideas for the design of 3D metal oxides and trigger the synthesis of other p–n heterojunction catalysts with a spatial structure.