Résumé : Using a double-plate wave maker, a series of laboratory experiments of internal solitary wave (ISW) loads on semi-submersible platforms were conducted in a density stratified fluid tank. Combined with experimental results, a numerical flume based on the Navier-Stokes equations in a two-layer fluid is developed to simulate nonlinear interactions between ISWs and a semi-submersible platform. The numerical results of horizontal and vertical forces, as well as torques on the semi-submersible platform also agree well with the experimental measurements. Besides, the numerical results indicate that the horizontal and vertical forces on the semi-submersible platform due to ISWs can be divided into three components, namely the wave pressure-difference forces, viscous pressure-difference forces, and the frictional force which is negligible. For the horizontal force, the wave and viscous pressure-difference components are of the same order, implying that the viscous effect is significant. For the vertical force, the contribution of the viscous pressure-difference is not important. Moreover, the diffraction effect is significant for horizontal force and insignificant for vertical force. Hence, it is feasible to estimate the vertical load using the Froude-Krylov approach.