Abstract:  In the South China Sea drilling operation, deepwater semi-submersible drilling platform experiences repeated wave interference, which causes a significant impact on equipment safety and platform operation. Considering the actual situation of internal waves acting on the semi-submersible drilling platform in the South China Sea as well as the semi-submersible drilling platform riser system used at present, the extreme motion condition of riser system is studied and analyzed. The maximum offset for the emergency disengagement of riser system, when the platform drifts as a result of internal wave, is put forward. Through the calculation example, the emergency disengagement conditions of semi-submersible platform under extreme conditions of internal waves are discussed. This research has important significance to the internal wave emergency management and the safety of deepwater drilling operation in the specific internal wave environment in the South China Sea.