[Objective] To explore the applicability of the combined forecasting model based on the third theory of quantification in combined landslide deformation and to provide a new idea for forecasting the combined landslide deformation. [Methods] We used GM(1, 1), support vector machine, BP neural network and MC prediction as a single prediction model of landslide deformation, and applied the third theory of quantification to analyze the importance of relative error interval and single prediction, and then determine the interval weights and the prediction error between groups, thus determining the combination weights of comprehensive prediction. [Results] The average value of relative error of combined prediction was 1.1% and the variance was 0.16. [Conclusion] Compare to single forecast model, the forecast accuracy and stability of combined prediction had been greatly improved.