Abstract: Evaporation duct plays an important role in electromagnetic wave propagation near the sea surface, and the prediction of regional evaporation duct and microwave path loss is very important for evaluating the performance of electromagnetic systems such as radar, communication and reconnaissance at sea. Firstly, based on Global Forecast System (GFS) prediction products and NAVSLaM evaporation duct prediction model, a large area evaporation duct prediction in the South China Sea is carried out in this paper. The reanalysis data of ERA5 published by European Centre for Medium-range Weather Forecasts (ECMWF) are compared. Then, combined with the parabolic equation model of electromagnetic wave propagation, the path loss prediction of microwave in evaporation duct channel is carried out. Finally, the prediction results are verified by the path loss test link in the northern part of the South China Sea. The verification results show that the path loss prediction in evaporation duct layer based on GFS data is in good agreement with the measured values, and in the first 24 hours, the error is less than 5−10 dB, which can be used to predict the path loss in large area evaporation duct layer. There is a large error between the forecast results and the measured data, which is caused by rainfall. When calculating electromagnetic wave propagation, the marine environment such as rain attenuation and rough sea surface should be fully considered.