Some limitations and insufficiencies are found in the detection methods to check the compactness of filled subgrade，so a new method was put forward. Firstly，in view of the characteristic that the solid(soil and rock) volume does not change in the soil-rock mixture subgrade when it is loaded，a model which can describe the change of porosity in the soil-rock mixture subgrade was established，and the changing regularity of the deformation modulus of soil-rock mixture subgrade was also deduced. Then，the soil-rock mixture subgrade was regarded as a semi-infinite elastic space，and the changing regularity of its deformation modulus under the action of load was taken into account，and a method for calculating the deformation or settlement of soil-rock mixture subgrade under the vertical surface load was brought forward on the basis of Boussinesq's solution by adopting the idea of step loading. Afterwards，the relationship between the initial porosity ratio of soil-rock mixture subgrade and load-moment curves was established，and the new way to detect the compactness of soil-rock mixture subgrade was set up on the basis of static load test curves. Finally，the site test data show that the results of the new method are close to that of the common one, indicating the feasibility and validity of the new method.