The more used forecasting method of post-construction settlement of subgrade is to analyze the measured data to determine the forecasting model parameters，and then use this model to forecast the post-construction settlement in actual projects. Currently used forecasting methods are Hoshino method, index curve method, hyperbolic method and so on, these methods have their advantages, disadvantages and applicability. To make the best use of the advantages and avoid the disadvantages, accordingly the combination forecasting method is proposed that can both comprehensive utilization the information provided in a variety of forecasting methods and also improvement of the forecasting accuracy. Then a superior combination forecasting model is established by the highest forecasting precision and the best forecasting stability. Taking the measured data of post-construction settlement of subgrade for example, comparison of fitting precision and forecasting precision of various forecasting methods , and show that the superior combination method has advantages of higher fitting precision, forecasting precision and their stability.