A modal testing and analysis system of tire vibration characteristics when the tire mounts on a free state are established in this paper, with the use of vibration experiment and modal analysis method. The order of frequency and the radial modal shape are extracted, the modal parameters of tire changing with the laws of inflation pressure is analyzed, through data processing and analysis of the radial tire excitation signal. It provides guidance for the research of experimental methods , tire structure design and dynamic performance analysis of vehicle.