Abstract: This paper describes the scheme of the horizontal interpolation from the Gaussian grid to regular latitude/longitude grid by using spectral method in the post—processing procedure of the T42 assimilation forecasting operational system in the National Meteorological Center (Beijing). The results show that the scheme has many advantages such as high interpolation precision and filtrating the short waves caused by the truncation error in the process of vertical interpolation. The scheme can provide more accurate forecasting values for users and first guess field for next assimilation