In this paper, the hydrodynamic interaction between Amirkabir semi-submersible and a Mono column FPSO floating platforms in Caspian Sea Conditions was studied. Dynamic responses of the floating platforms in the second-order Stokes incident wave was investigated using the boundary element method in ANSYS/AQWA software for both time domain and frequency domain. In this numerical simulation, the hydrodynamic loads acted on the hull body were determined by 3D radiation/diffraction theory, while the mooring line forces were estimated by the Morison Elements Method. The main purpose of the research is to study the effects of different distances between centers of platforms and different wavelengths on semi-submersible hydrodynamics motions, hydrodynamic forces, and mooring line tension. The numerical results show that as the platforms come closer, the values of diffraction force, motions and the oscillation of mooring line tension for the semi-submersible platform decrease. In addition, increase of wave length results in increase of the motions of semi-submersible platform. In this process the hydrodynamic forces and the oscillation of mooring line tension demonstrate a peak in wave length of about 55–60 m.