Abstract: To optimize the sensitivity of surface plasmon resonance (SPR) sensors, the reflectivity and sensitivity variances of bimetallic film SPR sensors with the change of silver-gold film thickness were studied based on thin film theory. The thickness of silver film and gold film had a certain range under the condition that resonance angle reflectivity was less than 1%. In the range of thickness, the sensitivity of bimetallic film SPR sensor increases with the increase of the thickness of silver film (or gold film). The increment of sensitivity was up to 5°/RIU. The results indicate that the sensitivity of bimetallic film SPR sensor can be improved by increasing the thickness of bimetallic film under the premise of certain resonance angle reflectivity.