In order to control the differential settlement when widening the existing roads,prediction of new base settlement is necessary.The paper selects the equal-interval insitu settlement value as the initial sequence,then adds the data from the first datum to the next to form the 1-AGO sequence,thus the differential equation is established.To solve this equation,the paper gets the settlement value alone with the variation of the time,and checks its precision.By analyzing the subgrade settlement data of Hejiaping Joint Line,the research proves that the settlement value predicted with GM(1,1) model is feasible.In order to gain more accurate results,the new updated data should be added to the model.