Rockfill dams are generally constructed on thick overburden layers in mountainous and valley areas due to their high adaptability to complex geological and geographical conditions. The safety of rockfill dams is a research focus worldwide, especially with the increase of dam heights. Significant differential settlement may be induced by different embankment materials in a dam body and various stratum properties in thick overburden layers. The accurate prediction of settlement is particularly important to guarantee the serviceability and safety operation of rockfill dams. Therefore, a novel settlement forecasting model is proposed in this study. This model can achieve a successful prediction of dam settlement during the construction and operation periods. Compared with the traditional model, the settlement influential factors in the proposed model are determined based on the existing theory of coarse-grained soil deformation calculation, causing the selection of factors to have clearer physical meanings. Additionally, the geometric characteristics of the dam, the different material properties in various zones of the dam and the foundation, and the complicated external loading conditions are all considered. Then, the improved regression model for forecasting the settlement of dams is proposed. The proposed settlement forecasting model is further employed to estimate the deformation of a high rockfill dam in China. The results prove that this novel method shows high accuracy and agrees well with the monitoring consequences, and this method is superior to the traditional statistical regression model in predicting dam settlement during the construction period.