Airgap is a crucial design consideration of semi-submersible floating structures, which is defined as the clearance between the platform deck and wave surface. Consequently, the platform motion and wave elevation are both important for airgap analysis. Furthermore, the nonlinearity of them enables airgap response to have evident non-Gaussian characteristics, which makes the prediction of extreme airgap response intricate. In this paper, a coupled analysis of the airgap response is performed. The software HydroStar is used to model the wave-structure interaction and construct the linear transfer functions and quadratic transfer functions for wave forces in irregular seastate. Then the platform motion and wave elevation are analyzed simultaneously in the time domain to get the airgap response, the nonlinear damping effect is incorporated during this process. Then an extreme analysis is carried out based on the time domain results. This study shows that the low-frequency (LF) roll and pitch motions of the platform increase the nonlinearity of the airgap response, especially for the points located at the edge of platform deck. Besides, the LF component of airgap response trends to increase the extreme value.