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In this study, we developed a kind of gold-silver alloy film based SPR (AuAg-SPR) sensors with wavelength interrogation to detect Cancer antigen 125 (CA125) by sandwich immunoassay. The experiment shows that the changes in resonance wavelength (Δλ𝑅) of AuAg-SPR sensors are higher than that of the gold film based SPR (Au-SPR) sensors when the final sandwich structures were formed. We have applied the Fresnel reflection equations and Bruggeman approximation formula to calculate the volume fractions of adsorbates in different adlayers, and the dispersion curves of the effective refractive index (RI) of adsorbates with different volume fractions of adlayers were obtained. Moreover, we simulated the sensitivity of RI of two kinds of sensors by using the waveguide equations, when the initial resonance wavelength (λ𝑅) was fixed at 628nm, changing RI from 1.333 to 1.335, the Δλ𝑅 of AuAg-SPR sensor is 2.25-fold compared with that of Au-SPR sensor. Finally, the effect of the size of antigen on the detection result and effective RI were analyzed. In conclusion, the AuAgSPR sensors with a higher sensitivity than the conventional Au-SPR sensors in detecting CA125, owing to the advantages of low-cost, stability, highly efficient for the clinical determination of CA125 levels
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