Coupling effects of surface plasmon resonance (SPR) induce changes in the wavelength, intensity, and linewidth of plasmonic modes. Here, inspired by coupling effects, we reveal an abrupt linewidth-shrinking effect in 2D gold nanohole arrays at the azimuthal angle of 45° arising from the interference of two degenerate SPR modes. We further demonstrate the biosensing capability under various excitation conditions for detecting the critical molecular biomarker of prostatic carcinoma, and achieve the maximum sensitivity at this angle. Our study not only enhances the understanding toward plasmonic resonance-linewidth shrinking, but also provides a promising strategy to greatly improve biosensing performance by light manipulation on plasmonic nanostructures.