In this paper, the surface plasmon resonance (SPR) technology for measuring the absorptive sample′s imaginary part of the refractive index is studied via numerical simulations. By using the Fresnel equation, intensity, phase, and angle modulations are analyzed. The results show that, the angle interrogation has the highest resolution (1×10-6 RIU) and the widest linear measurement range. So the angular modulation is a good choice for measuring the absorptive sample′s imaginary part of the refractive index.