Gold-silver bimetallic film configuration is brought forward to realize surface plasmon resonance imaging (SPRI) biosensor with the virtues of both high sensitivity and chemical stability. The theoretical calculation is adopted to optimize the thicknesses of the metal films, and bimetallic film configuration with high refractive index sensitivity and a good linearity between reflectivity and refractive index is presented. Then, the property of the detection system is discussed. The results show that in comparison to most commercial SPRI biosensors which use single gold films, the sensitivity and molecule detection ability of the gold-silver bimetallic film configuration can be improved to a great extent. For the substrate of BAK3 glass used in this paper, the sensitivity enhancement reaches as high as 80%, which makes it a much better choice for SPRI biosensing applications.