The invention discloses a method for predicting theoretical line loss of a regional power network. At present, in the calculation and the analysis on the theoretical line loss for guiding the loss reduction and energy saving and the planning of the power network, frequently, the current data is used, so that the analysis and the guidance cause big error, and the fineness of the line loss prediction is low. The technical scheme of the invention is characterized by comprising the following steps of: decomposing a year theoretical line loss rate into a plurality of typical period theoretical line loss rates according to the typical periods, and assigning corresponding weighting coefficients to each typical period; calculating the predicted values of the weighting coefficients and the theoretical line loss rates of each typical period over the year through a trend extrapolation method according to the data over the years; and finally, collecting the predicted values of each typical period over the years according to the decomposing characteristics to obtain the theoretical line loss rate of a target year. The method can accurately predict the theoretical line loss value of required year, and improves the effectiveness of the analysis and the guidance for the loss reduction and energy saving and the planning of the power network.