The key challenge in constructing molecular logic circuits is to design negative differential resistance (NDR) devices with high peak-to-valley ratios (PVR). This paper investigates the electronic transport properties of porphyrin molecules covalently connected on zigzag graphene nanoribbon electrodes via amide groups, using a first-principles calculation method combining density functional theory and non-equilibrium Green's function approach. The results show that porphyrin molecular junctions can achieve controllable NDR effects through metal atom modification. The intrinsic porphyrin molecular junction exhibits a symmetrical NDR effect under low bias voltage, with a PVR of up to the order of 10³. However, its peak current is limited by orbital localization to only 27.8 nA, restricting its logical switching performance. Modification of Au and Pt atoms significantly improves the spatial delocalization of the system’s transmission eigenstates and induces energy level degeneracy, thereby greatly enhancing the devices’ transport capability. As a result, the peak currents of Au-porphyrin and Pt-porphyrin molecular junctions increase to 238.1 nA and 258.2 nA, respectively. The Fe-porphyrin molecular junction forms a global delocalized channel under forward bias, with the peak current significantly increased to 611.8 nA while maintaining a PVR of 10³, achieving an optimal balance of NDR performance. This is confirmed by the evolution of the spatial distribution of transmission eigenstates, where the Fe-porphyrin molecular junction demonstrates pronounced asymmetric NDR behavior. This originates from the anisotropic exchange coupling between the unpaired 3d electrons of the central Fe atom and the delocalized π electrons of the porphyrin ring under an external electric field, thereby breaking the symmetry of the transport channel. In summary, the orbital space localization and channel decoupling caused by the Stark effect under low bias voltage are the common underlying mechanisms of NDR effects in all systems. The results provide theoretical guidance for the experimental synthesis and physical design of related functional devices in the future.