Revised Edlen′s equations by Boensch and Potulski in 1998 are mostly used to calculate air refractive index at present. Since the correction coefficient of water vapour influence on refractive index is performed with four wavelengths (644.0 nm, 508.7 nm, 480.1 nm, and 467.9 nm) and the temperature between 19.6 ℃ and 20.1 ℃, the accuracy will be influenced when the laser wavelength is 633 nm, which is mostly applied in optical precision measurement, and the environmental temperature is far away from 20 ℃. To solve this problem, a refractive index measurement system based on phase step interferometry is presented. The correction coefficient of water vapour influence on refractive index is measured and the revised equations are acquired with the wavelength of 633 nm and the larger temperature range (14.6 ℃~24.0 ℃). The comparison results show that the accuracy by presented equations is better than that by Boensch′s equations.