A surface plasmon resonance (SPR) sensor with self-adaptive structure for angle shift is presented. Combined with phase detection, a measurement system for the gas refractive index is designed. The theoretical analyses indicate that the factors which affect the response of phase detection and measurement precision are the incident angle of the laser beam, the thickness of the gold film and the nonlinearity between the phase difference of p and s polarization components of the reflected light and the refractive index of gas. The application of the self-adaptive SPR sensor reduces the measurement error by one-order of magnitude and greatly improves the sensitivity. The thickness of the gold film is designed and the error which results from the nonlinearity between the phase difference of p and s polarization components of the reflected light is evaluated as well. The comparison experiments of measuring the refractive index of air show that the accuracy of the gas refractive index measurement system reaches 10-6 order.