The principle of surface plasmon resonance (SPR) sensor is introduced. A SPR measurement system with Kretschmann configuration is designed based on wavelength modulation technology. The relationship between refractive index and resonance wavelength for ten different concentrations of alcohol solution is measured by detecting the light intensity reflectivities with fixing the incident angle while changing the wavelength. Experimental results show that the resonance wavelength increases with increasing sample refractive index, exhibiting good linearity. The SPR sensor can realize the measurement of refractive index or concentration of unknown solution by analyzing the resonant spectrum. The SPR sensor demonstrated here is featured with compact structure, simple operation, accurate and real time measurement.