A theoretical study based on the use of admittance loci method in the design of surface plasmon resonance (SPR) based structure using Ag-Au bimetallic alloy film of different alloy fractions and nanoparticle sizes has been reported along with some interesting performance related simulated results at 633 nm wavelength. The sensitivity and other performance parameter issues of the structure based on the choice of correct alloy fraction and nanoparticle size of Ag-Au bimetallic alloy film have also been discussed giving due importance to the dynamic range of the designed structure.