A high-accuracy absolute measurement system of the refractive index of air is proposed. The measurement of the instantaneous index of refraction is achieved by comparing two laser optical paths with a differential laser interferometer. The vacuum optical path length is varied via a motor-driven trapezoidal vacuum cavity moving through a known distance. Compared to other measurement methods, the sealed vacuum cavity requires no vacuum pumping between measurements, and allows this method to function well in industrial applications. The measurement accuracy is better than 5×10–8.