The first natural frequency of flexible deployable structures is mostly less than 2Hz. Low mode has caused difficulties to modal testing of such structures, because the average hammering test method can only stimulate the local modes rather than global mode of flexible structure. For flexible deployable truss structure described in this paper, a novel modal test method is proposed. The rope-spring gravity unloading system is used to eliminate the effect of gravity on modal. The ropes are set at 19 faceplates to unload the gravity at the same time. The flexible spring in the rope is used to release additional stiffness. When tested, flexible deployable structures are placed in horizontal and vertical directions in high-precision. The ultra-low frequency vibration exciter is the source of vibration. And laser displacement sensor is used for data collection. Both of them can increase the accuracy of the test results. Finally, the test results show that the vertical suspension method is more accurate. In addition, the test results are contrasted with that of finite element analysis, and it can be found out that two of them are approximately the same to verify the validity of the trial design.