Increasing demands for coupled seismic research of engineering structures and water bodies have promoted the construction and development of underwater shaking tables. The vertical hydrodynamic added mass and other characteristics of the tables affect their design and performance directly. This paper presents an experimental study of an underwater shaking table with an inertial mass block load of 20 tons under 135 working conditions of different inputs of excitation magnitudes

frequencies

and water depths

focusing on the characteristics of its vertical hydrodynamic added mass and on an analysis for preliminary explanation from the mechanism level. The test results show that a complicated correlation exists between the excitation magnitude and the hydrodynamic added mass

and grouping between different frequencies is observed. Water depth ratio (water depth over mesa diameter) is an important parameter to the design of a shaking table; different tables are different in the frequency inflection points at the minimum added mass. This study is a useful reference for the design of underwater shaking tables and the reliability of model tests.