Abstract: Atmospheric refractive environment over the sea is one of key factors on estimation of electromagnetic wave propagation and target detection. Evaluation on atmospheric refractivity over the South China Sea from ERA-I reanalysis data was carried out using the Globle Position System(GPS) radiosonde data. The results show that the global standard deviation and relative error of the modified atmospheric refractive index bellow 3 000 m heights is about 9.2 M units and 1.4% respectively. On altitude distribution, the modified atmospheric refractivity index is less estimated, displaying small-amplitude fluctuations in the lower atmospheric layer and larger estimated above the top heights of marine atmospheric boundary layer, which is closely related with the water vapor pressure distribution in the atmospheric layer and their correlation coefficient is 0.94. Moreover, for the duct parameters from ERA-I data, their duct heights are are consistent well with those from GPS radiosonde data, with their slope about 1 and their duct thicknesses are larger and duct strengths are weaker than those from the observed data, with each of their slope close to 0.5. The EAR-I reanalysis dataset over the sea is reliable and useful data source to study the marine atmospheric refractive environment.