Abstract: Frequent typhoons in the South China Sea may significantly affect safety of the operations on semi-submersible drilling platforms with anchor mooring system. By useing hydrodynamic calculation software, ANSYS-AQWA, the coupled dynamic analysis model for the platform-mooring system was established, and an analysis on the drift of semi-submersible platforms and the safety assessment of anchoring system were conducted based on three dimensional potential flow theory and Morison equation. The analysis results showed that when the mooring system was intact, both the average and maximum drift distances of platforms exceeded the allowable values specified in API. Under such circumstances, drilling operations should be suspended and a disconnection between the drilling riser and the wellhead should be done, if the anchor chain does not part. when the anchor chain of the least safety factor in the mooring system parted, the drift distance increased significantly, the minimum safety factor of the mooring system decreased obviously and in some cases, other anchor chains then parted in succession, leading to chain reactions. Research results showed that the proposed methods and the obtained conclusions could provide a reference for security assessment of mooring floating structures in extreme weather conditions.