Semi-submersible platforms are one of the important marine engineering equipment for marine resource exploration. The influence of column wave run-up, internal air gap and slamming on platform safety is the focus of domestic and foreign scholars. In this paper, the key problems of near-field interference, wave climb and air gap distribution of a semi-submersible platform were systematically studied by using model test method. Under the guidance of marine engineering hydrodynamic test regulations, the tank model test of the semi submersible platform in mooring mode was conducted. Through the deployment of optical non-contact six degree-of-freedom sensor, resistance type Tan wire, force sensor, wave height meter and other measuring equipment, the transient motion response of the platform, the wave run-up of the column wall and the air gap distribution in the internal field were obtained. The effects of different wave steepness parameters and scattering parameters on the air gap response characteristics of the platform were analyzed systematically. The results show that wave steepness has a great influence on the air gap distribution in the inner field, when the wave steepness parameter is kA=0.12~0.15, the amplitude of platform motion is large and the air gap at platform edge is small; due to the wave climbing effect of the column, the air gap near the column is smaller than the inner field. The research conclusions can serve as a guide for the platform design and the safe operation of the platform.