The internal solitary waves, have properties of two-way shear profile, significant velocity and acceleration, etc., which threaten the safety of deepwater floating systems in South China Sea (SCS), for the frequent occurrence and the high intensity of internal solitary waves in SCS. In recent years, offshore oil companies of China encountered many strong internal waves during its deepwater oil and gas exploration and development activities in South China Sea. However, the action mechanism of ISWs to floating structures is not understood clearly, and the internal solitary waves are classified as ocean current in API RP 2SK (3rd edition), therefore engineers ignore the velocity variance and long period “wave” characteristics in the design of floating structures. Furthermore, the offshore floating structures is oscillating under the action of environmental forces, due to the horizontal velocity of the platform is comparative to that of ISWs, thus fluid-body coupling is significant that one cannot analyse it by simply adding a specified ISW force time history to the floater. This paper proposes a new iterative updating method for ISW loading calculation considering the fluid-body coupling, and applied this method to a semi MODU, numerical study shows the iteration is efficient and the result is more reasonable compared to conventional method, and it is found that the maximal offset decreases significantly.