A measurement system of refractive index of air (RIA) based on surface plasmon resonance (SPR) and phase detection is presented. A heterodyne interference optical path is built and a SPR sensor with self-adaptive structure for angle shift is applied. Theoretical analyses indicate that the variation of the phase difference between p and s polarization components of the measurement signal versus that of the reference signal is almost linear with RIA fluctuation. Thus the formula of RIA measurement is derived. The application of the self-adaptive SPR sensor reduces the measurement error by an order of magnitude and greatly improves the sensitivity. The comparison experiments with the method of Edlen equations show that the measurement accuracy reaches less than 5×10-6 at the phase detection precision of 0.1° and the incident angle of 44.0° (which is close to the resonance angle). Furthermore, the measurement system can provide an initial estimate of RIA accurate enough for other measuring instruments with higher precision.