In order to study the coupling vibrations of low-speed maglev trains moving and standing still on simply-supported girders, dynamic experiments were carried out on a 25m PC simply-supported girder of Changsha Maglev Commercial Line. The accelerations of the maglev car body, suspension frame and the simply-supported girder were measured when the maglev train passing and standing still with different weights (heavy vehicles: 30t + 30t + 30t, light vehicles: 24t + 24t + 24t). The test data show that when the maglev train running at low speed, the suspension system is unstable leading to the acceleration of the simply-supported girder with large peak at 60Hz, and the amplitude decreases with the train speed. The accelerations of the simply-supported girder at low speeds is larger than that at relatively higher speeds and decreases with the speed in the low speed range. The frequency (60Hz) is not affected by the suspension weight. The bigger the suspension weight is, the more unstable the suspension system is. The stable speed is 50km/h for the heavy train and 30km/h for the light train. The instability effect on the accelerations of the car body and the suspension frame increase with the suspension weight. When the maglev train stands still, the girder is subjected to the continuous force with excitation frequency of 90Hz, so the accelerations of the car body, suspension frame and the simply-supported girder all have predominant peaks at 90Hz. However, the predominant frequency (90Hz) is not affected by the suspension weight.