A gas pressure measurement device is built based on the gas refractive index method, and the measurement range is from 10 kPa to 100 kPa, and the device is tested by a dual-cavity comparison method. The temperature of the two vacuum chambers is separately controlled and the two cavities are connected to ensure that the gas pressures in the two cavities are the same. Using high-purity nitrogen (6N) as the gas medium, under the condition of different gas pressures, the dual-cavity comparative measurements are conducted. The results show that the optical pressure gauge has a repeatability better than 30 × 10^–6, which is significantly better than the commercial capacitive pressure gauge, indicating that this method has great potential applications. Sources of error and uncertainty in the measurement are analyzed, and it is planned to design a new system to improve the measurement.