As semi-submersible structures have smaller resonant frequency in the heaving and rolling mode compared to surface vessels, the response of a freely floating semi-submersible structure for Tsunami is studied experimentally. The solitary wave is chosen as Tsunami. The maximum displacement and inclination of the structure show easy result to understand when the maximum slope of the solitary wave or the ratio of wave height to water depth is taken as a parameter like the wave period in regular waves. Approximately speaking, the structure climbs up to the wave height, moves horizontally as the same to the water particle, and inclines to the maximum slope of the solitary wave. The structure, however, climbs up and inclines a little more than those when the height-depth ratio is small, while a little less when the height-depth ratio is large.