Abstract: The problem of air gap plays an important role in the concept and structure design of semi-submersible platform. For the common business software can not get result with nonlinear waves and Second-order motion directly, this paper forecast the air gap of a semi-submersible rig in the south China sea by a self-programming. Comparing with the results of the first order and second order air gap, the paper discussed the influence factor, and give the second order air gap response of each wave direction.