Composite foundation is commonly adopted in soft foundation treatment to reduce the differential settlement between new and old subgrade and the bumping at bridge-head in expressway reconstruction and expansion. Through grey relation entropy method, the author analyzes the influence of various factors on settlement during subgrade filling and the sensitivities of soft soil subgrade settlement under composite foundation treatment.  Results show that the settlement during subgrade filling which accounts for the final settlement, is influenced successively by thickness of surface crust, height of embankment filling, surface compression modulus, soft soil layer compression modulus and the width on embankment top, depth of soft soil, filling time, and thickness of soft soil layer. This research is of guiding significance for the design of soft foundation treatment and the monitoring during construction.