Measuring refractive index of air based on laser synthetic wavelength interferometry. The Apparatus includes a dual-frequency laser that emits orthogonal linear polarized light of wavelengths λ1 and λ2, a beamsplitter, two polarizing beamsplitters, two corner-cube retroreflectors, a quartz vacuum cavity of length L disposed in the measuring optical path in parallel to the light propagation direction, and two detectors. The apparatus is used to measure the refractive index of air using the dual-frequency laser to emit orthogonal linear polarized light with wavelengths λ1 and λ2, using the beamsplitters, corner-cube retroreflectors, quartz vacuum cavity, and detectors. The integer N and fraction ε of interference fringes of wavelength λ2 are determined. The refractive index of air n is obtained by using the length L of the vacuum cavity, integer N and fraction ε of the interference fringes of wavelength λ2. The measurement is accurate up to 10−9 or higher, and has strong anti-disturbance ability to the environment.