To provide a low-cost simulated excitation source at ultra-low frequencies at or below 1.0 Hz for investigations of the low-frequency dynamic characteristics of large lightweight structural components, an ultra-low-frequency excitation system based on an industrial six-axis robotic arm was designed, and its communication architecture, trajectory-planning method, and excitation performance were evaluated. High-density discrete trajectory points were directly transmitted from the host computer to the lower-level controller of the robotic arm via the TCP/IP protocol. Sinusoidal excitation in the 0.1~1.0 Hz range was generated by integrating local velocity feedforward with an amplitude-constraint strategy. The displacement response of the end effector was measured using a laser Doppler vibrometer. The experimental results showed that a favorable command-response linear relationship was achieved in the ultra-low-frequency range, with high frequency-tracking accuracy and continuous, stable output waveforms. Further amplitude-frequency characteristic tests indicated that the system satisfied the basic requirements for ultra-low-frequency displacement excitation in ground dynamic tests of typical large lightweight structures. Finally, the engineering feasibility of the excitation system was verified through semi-active vibration suppression experiments on representative flexible members.