Abstract: Hydropower energy is one of the most important clean energy sources in the world. Since the medium and high head hydropower energy presents a high economic index and exploitability, only a few resources are available at present in China. By contrast, low head hydropower resources become one of the key directions of hydropower development in the future. A Kaplan turbine has also been one of the main unit types for the low head hydropower resources. Nevertheless, cavitation has been only one factor to limit the application, although a wide range was achieved in the high efficiency region. Furthermore, the runner erosion and vibration induced by the cavitation can greatly shorten the normal operating cycle of the Kaplan turbine, even seriously detrimental to the economic benefits of water stations. The vibration and pressure fluctuation are the commonly-used indicators to report the stability of hydraulic turbines. As such, the effect of cavitation on the vibration and pressure fluctuation directly determines the stability and optimization of the Kaplan turbine. In this study, a synchronous test system was constructed to investigate the influence of cavitation on the pressure fluctuation of the draft tube and the runner vibration in a Kaplan turbine. The system included a high-speed camera, a Laser Doppler Vibrometer (LDV), and a high-frequency pressure fluctuation sensor. A Kaplan turbine model was also selected as the research object. The pressure fluctuation, radial vibration, and cavitation images of the runner were first captured under different cavitation coefficients in the synchronous test system. The Variational Mode Decomposition (VMD), the spectrum analysis, and the cross-correlation were then combined to explore the influence of cavitation on the radial vibration of the runner, and the pressure fluctuation on the draft tube. Finally, it was found that there was a significant increase in the amplitudes of pressure fluctuation on the draft cone tube, and the radial vibration velocity of the runner, with the decrease of cavitation coefficient. There was an obvious nonlinearity in the peak-to-peak value of pressure fluctuation on the draft cone tube and the radial vibration velocity of the runner, with the variation of cavitation coefficient. Specifically, the peak-to-peak values after complete cavitation reached 9.16 and 10.12 times that without cavitation, respectively. The increased severity of runner cavitation made the dominant frequency of pressure fluctuation on the draft cone tube change to blade passing frequency. But only a little influence was observed on the dominant frequency of runner radial vibration velocity. The dominant frequency of radial vibration velocity of the runner was basically the passing frequency of blades. The runner cavitation also induced the local migration of extreme value in the high-frequency energy, which increased the energy of low-frequency region of pressure fluctuation. The possibility of Kaplan turbine resonance was enhanced in this case. Moreover, the cavitation contributed to enhancing the energy in the high-frequency region of runner radial vibration in a Kaplan turbine.