The grey control theory is used to predict electric power demand in this paper. Original data is processed by the Generation Method. Many unimportant factors affecting electric power demand are removed,and useful information is extracted from original data. The differential fitting equation is set up,and grey prediction model modified by slip average method is presented with residual modification. The current year data is possessed with high weight,which avoids excessive fluctuation. Predicting results show that the model is effective to improve the predict precision.