The evaporation duct is a natural microwave trapping phenomenon over water surfaces, which has a strong impact on near-surface electromagnetic propagation. For radar and communication system designs and operations,it is of great significance to obtain the long-term mean features of evaporation duct in a large-scale sea. In order to overcome the shortcomings of single-point measurement at local sea, a method for calculating the statistical features of the evaporation duct in a large scale-sea based on the NCEP （National Centers for Environmental Prediction） reanalysis data was presented. This method was confirmed by using the weather buoy data at the Pacific Ocean and the measurement data on the Weizhou Island in the South Sea of China. With NCEP reanalysis data of 18 years, the spatio-temporal features of evaporation duct in the West Pacific Ocean were studied. The distribution features of the evaporation duct height for different seas and months were obtained. The evaporation duct database was established with spatial resolution of about 1875°×19°, which is 28 times higher than the existing 10°×10°database.