Aiming at the problem that the mooring stiffness is too large in shallow water and the mooring line tension tends to increase sharply in extreme sea conditions, by the numerical method, the cause of the abrupt increase of the mooring line tension is studied, and the influence of the groundline length of the mooring line on the maximum horizontal offset and the maximum tension of the mooring line of a semi-submersible platform under two different water depths is analyzed. The influence of seabed parameters on the response characteristics of the mooring system is considered. The results show that: the excessive mooring stiffness and strong nonlinearity are the important reasons for the abrupt increase of the mooring line tension in shallow waters; increasing the length of the groundline of the mooring line can reduce the axial stiffness, effectively change and optimize the total horizontal stiffness of the mooring system, and reduce the mooring line tension amplitude. The scheme to optimize the groundline length of the mooring line can be applied to moored floaters in shallow waters.