We present a refractometer that is capable of measuring the refractive index of gases with an unambiguous range of 1.000395 and an uncertainty of 3.14×10(-8) at 633 nm. The measurement range was extended via the combination of the vacuum cells according to the proposed synthetic pseudo-wavelength (SPW) method. The basic principles of the SPW method and the design of the gas refractometer are presented in detail. The performance of the refractometer was verified in the measurements of dry air, nitrogen gas, and ambient air under different environmental conditions. No gas-filling or pumping processes were required during the measurements; so one measurement could be completed within 70 s. Compared with existing refractometers, the method reported here holds advantages in its large unambiguous measuring range, fast speed, high accuracy, and simple instrumentation design.