This paper introduces a field test of retailing backwall new filling subgrade reinforced with cement-gravel pile of expressway in Hunan province. The change law of subgrade resilient modulus and data of heavy dynamic penetration were analyzed and contrasted, the tests results indicated that construction of cement-gravel pile can increase subgrade resilient modulus and composite subgrade can meet requirement of high grade highway with resilience modulus greater than 30 MPa. The main rebound deformation were produced in the first and last unloading in loading test of composite foundation，and other unloading performed similar, its deformation property was similar to the soil-rock subgrade.The composite modulus, predicted by the mean value of total rebound deformation or middle-level rebound deformation in the composite subgrade loading test, are greater different to predict under the replacement rate of pile-soil area. The construction of cement-gravel pile increases the soil strength of deep roadbed, but the disturbance and destruction of roadbed soil occur in the range of 1.0~ 1.5m under the pile top due to the large construction energy and insufficient overburden soil pressure.