The Bailongjiang water diversion has been determined as one of the 172 large-scale water projects by the government, which can be offered a technological support by analyzing the runoff variation characteristics of the representative stations on the mainstream of the Bailongjiang River and predicting the change trend. Based on the observed runoff data from the 4 representative stations (Baiyun, Zhouqu, Wudu and Bikou) on the mainstream of the Bailongjiang River from 1956 to 2013, we made an analysis and forecast of the change tendency of the runoff by using an additive model combined with 3 models (the model of periodic wave mean epitaxial superposition, the model of harmonic analysis and the model of stepwise regression analysis). The results show that (1) the long-term average flow in September usually takes the highest percentage of the total in the whole year at the 4 stations, the long-term average flow from June to September often reach 50% of the total in the whole year; (2) there is a decreasing trend of the mean annual discharge at the 4 stations, of which reduction is less at the upstream stations (Baiyun, Zhouqu) than that at the downstream stations (Wudu, Bikou); (3) the prediction results of the annual runoff in 2015, 2020 and 2025 is that the runoff will be less than the long-term average runoff at the Baiyun station, more than the long-term average runoff at the Wudu station, and the annual runoff will float up or down the long-term average runoff at the stations of Zhouqu and Bikou.