In order to overcome two faults of the complicated calibration experiment in present methods for quick testing subgrade compactness and the neglectful effect of initial ground stress on deformation parameters, a new method for quick testing subgrade compactness was established. Firstly, the laws of porosity, deformation modulus and density varying with the deformation of subgrade soils were studied based on porous media theory. Then, the force-settlement analytical relation was established by considering the effect of initial ground stress and subgrade deformation on variational mechanical parameters by using the layer-wise summation method and step-loading idea in the process of static penetration. Besides, on the basis of force-settlement curves observed by static penetration experiments, the parameters of force-settlement analytical relation were obtained by back-analyses with adaptive genetic annealing algorithm, and the determination method for subgrade compactness based on porous media theory was then established. Finally, the engineering cases proved that the accuracy of this method satisfied the engineering requirements, and this method tested the subgrade compactness quickly and accurately without complicated calibration experiments which are essential in present methods. It shows that the proposed method is reasonable and feasible.