The employment of spin polarization under an external magnetic field holds great potential for the improvements of photocatalytic performance. However, owing to the huge difference in dielectric properties between ferromagnetic oxide and polymers, the photogenerated excitons with spin states are often limited to the ferromagnetic oxide wells, which leads to unsatisfactory activity. In this paper, a single-atom Co-doped C₃N₄ photocatalyst is successfully synthesized for photocatalytic water splitting and simultaneous oxidation of benzylamine. Under a tiny external magnetic field (24.5 mT), the hydrogen production rate could reach at 3979.0 μmol·g⁻¹·h⁻¹, which is about 340 times that of C₃N₄. Experimental results and theoretical calculations indicate that the interaction of Co d and N p orbital changes the symmetry center of C₃N₄, resulting in an increase in dielectric constant and spin polarization. Moreover, magnetic fields further promote parallel electron spin, and the increased number of charges with the parallel spin-down state is likely to dissociate under the action of an external magnetic field. On the other hand, the Co–N bond provides a huge built-in electric field and active site for strengthening the charge transfer and surface reaction. This work not only deepens the understanding of spin polarization, but also enriches methods to accelerate electron–hole separation.