Modal testing is essential to correct the structure of dynamics model and optimize structure designs. First, the principle and the application of modal testing system are introduced in this paper. Then, through the theoretical calculation, the frequency of the air spring is achieved in this suspension system. And the influence of the baric fluctuation to the rigid suspension system is analyzed. In order to achieve an ultra low frequency request at 0.03Hz, methods of reducing the baric fluctuation are proposed here. Through the analysis, we successfully design a modal testing suspension system which can test the natural frequency of the samples below 0.1Hz. Experimental results show the baric fluctuation of the suspension system is lower than 20Pa.