The modal parameters are the key indexes for the damnification diagnosis and state evaluation. But for the complexities of the structure and soil interaction effect mechanism of cantilever retaining wall system, there is still no appropriate ways to simulate the modal parameters effectively at the moment. To solve the existing problem, practical modal parameters of structure system were obtained by field impulse hammering modal test. Compared them with the modal parameters of structure system ignoring the soil influence, it was concluded that the soil influence can not be ignored. And with the consideration of soil influence, calculation methods of additional stiffness and mass were analysed and modal analysis with the finite element method was undertaken for the retaining wall as well. It was approved that the calculating method for soil influence and impulse hammering modal test is feasible with good agreement with data from field test. Finally, the main disabled modes of the retaining wall, the modal parameters characteristics under damage state and health diagnosis methods were studies and a fast, simple, lossless and effective modal analysis evaluation method was proposed for the health diagnosis of cantilever retaining wall.