Abstract: The new meteorological satellite named Fengyun-3C was launched on 23 September 2013, which loaded the Global Navigation Satellite System Occultation Sounder (GNOS). GNOS can be used to sound the neutral atmosphere and ionosphere, providing the profiles of the refractivity, density, temperature and humidity. Before the GNOS products are used, it is necessary to assess their accuracy. When compared with the collocated measurements of radio sonde, the bias and the standard deviation of the GNOS refractivity are about 0.25% and 2%,respectively,from the near surface to 25 km; while the temperature bias and standard deviation are 0.5 K and 2 K, respectively. As for the specific humidity, the standard deviation is about 0.25-1.0 g/kg below 6 km. The validation shows that GNOS can basically meet the design requirement, which indicates an important improvement to the first trial of the Fengyun satellite in the radio occultation sounding area.