Multiferroic tunnel junctions (MFTJs) exhibit rich spin transport behavior through the coupling between magnetism and ferroelectricity. While tunneling magnetoresistance (TMR) is commonly tuned by reversing the ferroelectric polarization ($P_z$), frequent switching undermines device stability. Using density functional theory, we reveal that spin transport in a $\mathrm{SrRu}{\mathrm{O}}_3/\mathrm{BaTi}{\mathrm{O}}_3/\mathrm{SrRu}{\mathrm{O}}_3$ MFTJ model can be effectively modulated through the coupled effects of band symmetry, in-plane strain (${\delta }_{xy}$), and $P_z$, without flipping polarization. At ${\delta }_{xy}=0\%$ and $P_z=11.68\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$, the TMR reaches 87.6%, surpassing the previously reported 75%. Under the conventional TMR definition, these values correspond to $\sim 1413\%$ and $\sim 600\%$, indicating a more than twofold enhancement. The tunneling spin polarization (TSP) also shifts from −80% to 57.6% as ${\delta }_{xy}$ varies from −3% to 2%, with a sign reversal near ${\delta }_{xy}\sim -2\%$ and $P_z=35\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$. Detailed DFT analysis reveals that these effects originate not from dielectric effects along, but from symmetry-dependent responses of wave functions to ${\delta }_{xy}$ and $P_z$. Our work uncovers the missing symmetry factor in strain-polarization-TMR coupling and suggests a pathway toward strain-sensitive, energy-efficient spintronic and neuromorphic devices.

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• Electric polarization
• Electrical conductivity
• Ferroelectricity
• Magnetotransport
• Spintronics