Aiming at the issue that the air refractive index measurement accuracy with the Edlén equation is limited by the sensors' accuracies and the integral interference fringe number is difficult to determine using laser interferometry with a length fixed vacuum cavity, an air refractive index measurement method combining laser single-frequency interferometry and PTF sensing is proposed. We design a sinusoidal phase modulated laser single-frequency interferometer with one length-fixed vacuum cavity for measuring the air refractive index. A pre-estimated value of air refractive index is obtained to determine the integral interference fringe number using the environmental parameters obtained by the low precision sensors. Then, the PGC-Arctan algorithm is adopted to accurately demodulate the phase of the interference signal to obtain the fractional interference fringe. Therefore, real-time large-range and high-accuracy measurements of air refractive index can be realized. The experimental setup is built, and the measurement results of the proposed method are compared with the findings of the Edlén equation. The experimental findings show that the measurement results of the two methods are in good agreement. The standard deviations of the differences between the measurement results of the two methods in 12 min and 1 h are 1.5 × 10^-8 and 2.3 × 10^-8, respectively. Experimental results indicate that the proposed method can be applied to the real-time compensation of air refractive index in laser interferometric precision displacement measurement.