The mooring system is designed for a semi-submersible drilling platform designed for the operations in polar seas. The mooring positioning capacity of the platform is analyzed and checked. The water depth of the design is in the range of 100 m to 500 m. The 1-year environment is selected as the design condition of the operating mode, and the 100-year environment is picked for the survival mode. To verify the positioning ability of the mooring system in polar sea environment, both weak ice and ice-free conditions are analyzed. Wind, wave and current loads are selected as the main environmental loads in the non-glacial period, and wind, current and ice loads are picked as the main environmental loads in the initial and final ice stages. The results show that if the harsh conditions such as icebergs are not considered, the conventional mooring chain can be used as the mooring system, which can meet the operation requirements of the platform in the summer and weak ice periods in the polar seas.