Abstract：Based on interpretation of the monitoring data of actual displacement which resulted from the landslides in Xintan，it is shown that the stability of whole homogeneous colluvial slope is closely correlated with two parameters，velocity of surface displacement and angle of surface vector. The stability trend can be described and evaluated by both the displacement velocity and the vector angle. The displacement vector angle，especially，for which there is no substitute，is an important parameter serving as an explicit criterion for the stability of slope，and hence it is significant for prediction of landslides. So systematical space-time analysis is made on the features of vector angles of surface displacement. From this analysis，the distribution laws of space-time and the characteristics of the displacement vector angles are deduced. On the basis of these results and using the principles of statistics，thd double-parameter criterion for forecasting the stability of the colluvial slopes is established. According to the double-parameter criterion，calculation and evaluation of stability are made on Xintan slope by monitoring data on F-series of points on the slope. The prediction results are coincided with the actual time and the laws of the landslide，which demonstrates that the double-parameter criterion is of satisfactory precision and practicality in prediction of landslides.
