For a quantitative analysis of the detection performance of wavelength surface plasmon resonance sensor after the introduction of absorbing thin films, a mathematical model was established for the wavelength SPR biosensor. Based on absorption theory of dispersive media, the distribution of electromagnetic field in each film outside the prism is calculated. The results show that, after introducing the absorbing thin films, electromagnetic field energy distribution in each layer out of the prism surfaces was changed, the metal film contribution on the resonance absorption was reduced, which increases the half wave width of SPR curve and the minimum reflection coefficient, so the optimal film thickness of metal film was also changed. Then the effect of colloidal gold and immunogold on SPR biosensor has been theoretically and experimentally verified. The result shows that, compared with the 48 nm gold film SPR sensor, the sensitivity increases 1.5 times when 45nm gold film is surface-modified with 10nm nano-gold.