Refractive index is an important parameter who indicates the material information. In chemistry, biology, medicine and other fields, the samples are usually made of low concentration solutions for chemical or physical reactions, from which various properties and parameters of the samples are determinated by detecting small changes of the refractive index of the solutions. So that, a kind of laser measurement method for liquid refractive index is presented based on surface plasmon resonance (SPR) and phase detection. The theoretical analyses indicate that the phase difference change between p and s polarization components of the reflected light is almost linear with the fluctuation of the refractive index at the range of 1.333~1.336. Thus the measurement equation is derived. Corresponding refractive index measurement system is built based on a heterodyne interference optical path using a dual-frequency laser. The system has strong anti-interference ability because of the common light path and high precision because of using phase measurement. The experimental results of measuring glycerine solution refractive index are consistent with the theoretical analysis, and agree with the results of another method, where the error between each other is less than 2.0×10-5. The method can be widely applied in the fields of biological detection, chemical analysis, medical diagnosis, pollution research and so on.