The key challenge in constructing molecular logic circuits is to design negative differential resistance (NDR) devices with high peak-to-valley ratios (PVR). The paper investigates the electronic transport properties of porphyrin molecules covalently connected on zigzag graphene nanoribbon electrodes via amide groups using first principles calculation method combining density functional theory and non-equilibrium Green's function. The research 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, which restricts its logical switching performance. The modifications of Au and Pt atoms can improve the spatial delocalization of the transmission eigenstates of the system and induce energy level degeneracy significantly, thereby greatly enhancing the transport capability of the device. Therefore, the peak currents of Au-porphyrin and Pt-porphyrin molecular junctions can be increased to 238.1 nA and 258.2 nA, respectively. Fe-porphyrin molecular junction forms a global delocalized channel under forward bias, and the peak current can be significantly increased to 611.8 nA while maintaining a PVR of 10³, achieving the optimal balance of NDR performance. In particular, confirmed by the spatial distribution evolution of the transmission eigenstates, 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, which breaks the symmetry of the transport channel. In summary, the orbital space localization and channel decoupling caused by Stark effect under low bias voltage are the common underlying mechanisms of NDR effects in all systems. The research results provided theoretical references for the experimental synthesis and physical design of related functional devices in the future.