Abstract: Transformer vibrations indicate their operating status and health, which subtly affects the system stability and reliability. In this paper, linked to a flexible AC transmission project, a simulation model was created for a 20 Hz low-frequency transformer during no-load operation, considering the "vibration source-oil-tank" coupling across multiple physical fields. Moreover, the transformer's vibration traits were studied and the simulation accuracy was confirmed with on-site measurements of the transformer's current and the tank's surface vibrations. Findings show that, in view of decreases in the tank's vibration acceleration in the process of fluid-solid interaction, the damping effects can more precisely mimic the vibrations. Compared to conventional transformers, the 20 Hz low-frequency transformer has smaller harmonic amplitudes in phase current with no dominant frequency; the vibration spectrum mainly focuses on the double-frequency of 40 Hz, with a notable vibration peak at 160 Hz in the higher spectrum. For vibration monitoring, the middle point on the tank corresponding to the core column in the lamination direction is most significant, which can ensure the collection of vibration signals without interference from components such as stiffeners. This research supports the future condition assessment of transformers in offshore flexible low-frequency applications.