The article puts forward that the observation time and single observation result have large effects on the final settlement which is predicted by conventional forecasting methods based on analyzing Zhengzhou-Xi’an passenger dedicated line test section subgrade settlement. As a result

predictive value may not real value. The article puts forward that the development of subgrade settlement is an easing up process. It can be significantly divided into consolidation deformation stage and rheological deformation stage based on the bases of analyzing subgrade settlement deformation curve. The viewpoint can be explained by equivalent consolidation theory. The article puts forward the method for predicting final settlement according to s- curve characteristics. The practice shows that the observation time and single observation result have very small effects on the final predict settlement. It is better than any other methods.