Abstract: This paper decides the discipline of soft soil subgrade lateral deformation after the approach of plastic drainage plate in the southwest area. Based on the project of expressway from Suining to Ziyang in Sichuan Province, we sort out the typical observation data of monitoring sections deformation. We analyze the variation law of lateral deformation from the angle of time and depth. Meanwhile we propose the forecasting model according to the characteristic of deformation. The results indicate that the following.(1)During the process of filling, the lateral deformation near the ground surface alters violently. (2)The lateral deformation distributes three variation laws: during the filling stage, the relationship between the lateral deformation and time is ym=SD. The height from the critical filling to the design process, the relationship is ym=0.15SD. In the consolidation stage, it is ym=0.3SD. The maximum lateral deformation is distributed as S-shape with time. The maximum deformation position does not change. (3)We propose and predicte the y(z)=ae-z(bsinz+ccosz)、y(t)=a/(1+e(b-ctr)) curve according to the variation law of lateral horizontal deformation from the angle of depth and time. The result indicates that the predicted life matches the real data.