In order to meet the construction requirement of cross-sea bridges, the semi-submersible barge is converted into a construction platform. In order to understand its wave dynamic characteristics in the floating state, a three-dimensional full-size mathematical model of the wave and modified jack-up platform is established based on the potential flow theory. Through the hydrodynamic calculation in the time domain, the platform motion response at different pile leg lowing depths and different wave directions is analyzed when the modified jack-up platform is in the floating state. Through calculation, it is found that: when the platform pile legs are lowered at different depths, the modified platform as a whole is in a stable state, and its overall motion response is small compared to the size of the platform itself; under the action of waves in different wave directions, the motion response amplitude and motion trajectory of the modified platform in the horizontal direction and vertical direction are different, and during the process of the pile legs lowering, efforts should be made to ensure that the platform has a small wave facing area as much as possible. The conclusion obtained is of certain guiding significance for the overall stability study of the deepwater jack-up platform in the floating state.