Based on the geologic data of a existing expressway，for its widening engineering，the improvement effect of new and old road foundations with different improvement methods is simulated numerically by popular FEM program. Result shows that the loading of new embankment will induce the instant deformation of old road to some extent; and the new road foundation with composite foundation method will produce less differential settlement; and the consolidation degree of old road foundation before constructing new one has less effect on the improvement effect of composite foundation method.