Abstract: How to control of the differential settlement is a key task for the soft soil subgrade in Expressway extension project. This paper examines the experiment section of one expressway extension project in northern China. The finite element calculation is carried out for the deformation of widening soft soil subgrade. The Abaqus software is used. The calculation results are in accordance with the deformation monitoring information at field tests. These results conformably demonstrate the deformation characteristics that widening subgrade presents larger settlement rate in the initial stage of embankment filling. But the old subgrade has obvious additional settlement. The basin-curve of settlement appears in the bottom of widening subgrade. The differential settlement between the new and old subgrade is one of the main reasons that inducing longitudinal subgrade cracks. The foundation soil near the bottom of the composite foundation of prestressed thin-wall concrete (PTC) pile with cap presents obvious extrusion deformation in horizontal directions, which shows that PTC pile tip should be placed at effective bearing stratum with relatively high capacity.