To salve the influenees of the tilt of optical windows and the residual air pressure in the vacuum cavity on the measurementaccuracy in the laser interferometrie air refractive index measurement based on the variable length vacuum cavity, this paper analyzes theerror caused by the optical window tilt when the vacauum cavity is pulled. And a method for detecting and compensating the tilt ermor ofwindows is proposed. In addition, the method of measuring and compensating the air residual pressure in the vacuum cavity is studied.To verify the effeetiveness of the proposed method, a laser interferometrie air refractive index measurement system based on the variablelength vacuum cavity is constructed, the measurement results with proposed method are eompared with the results obtained by Edlénequation, Experimental results show that, the measurement results of the two method are in good agreement. In 2.2 h and 20 min,thestandard deviations of the differences between the measurement results of the proposed method and that of the Edlen equation are 2.34×10and 2.89×10⁻°,respectively. This indicates that the proposed measurement error eompensation method of air refractive indexis feasible and effective, which can be applied to the real-time compensation of air refractive index in laser interferometrie precisiondisplacement measurement.