The invention discloses a power transmission line icing long-term and quantified forecasting method. The method comprises the following steps of: 1, collecting history circulation index data and icing data; 2, calculating coefficients related to circulation indexes and an average icing day number, and preliminarily selecting icing forecasting factors; 3, utilizing a discrete particle swarm algorithm to obtain the icing forecasting factors highest in forecasting performance; 4, based on the selected forecasting factors, utilizing the history data to establish a least square regression prediction model; and 5, utilizing the established model to carry out quantified forecasting on the icing degree of a power grid in winter. The method has the advantages that 1, quantified forecasting of the icing can be carried out a month in advance on the power grid in a future quarter (winter); 2, the operation performance is high; 3, the accuracy for power transmission line icing forecasting is high; and 4, the problem of power grid icing long-term and quantified forecasting is solved. According to a predicted result, the manual power and the material for handling the icing of the power grid can be scientifically and reasonably planned, related economic disposition can be performed, the icing of the power grid is handled in advance, and the loss caused by the icing of the power grid is lowered.