Based on observational data, the stability in the empirical orthogonal expansion of the drought and flood indexes is analyzed, Results show that there have been no significant long-term trend and periodic oscillation in the eigenvectors during the last 500 years. It is also found that the stability in the structure of the eigenvectors decreases with their orders, whereas that in the characteristic value of the eigenvectors increases with their orders. Considering the stability in the spatial structure and variance contribution of the eigenvectors, only the first 4-5 eigenvectors are of climatological significance