Based on the characteristics of compound permittivity of metal film and the perturbation to wave-vector of surface plasmon wave,the full wave half maxium of optical spectrum has been investigated under the surface plasmon resonance sensors in wavelength interrogation.A mathematical expression has been established to describe the relation between the FWHM and parameters of the SPR sensors.The oretical research is consistent with previous experimental results and it could be employed to be theoretical guidance in optimizing properties of SPR sensors.