The invention provides a method for predicting medium and long term tendency of electric energy quality indexes. The method comprises the following steps of determining the electric energy quality indexes; calculating an electric energy quality index differential value; and predicting the medium and long term tendency of the electric energy quality indexes. The invention provides a method for predicting the medium and long term tendency of the electric energy quality indexes based on exponential smoothing, which is used for judging the medium and long term tendency. The method is obvious in index effectiveness and good in intuition, complete in electric energy quality index coverage, wide in application range, and high in transplantability; and the electric energy quality indexes can be calculated by periods and classes.