With the entry into force of the Ballast Water Management (BWM) Convention, the demand for the ballast management system of large semi-submersible vessels becomes increasingly urgent. It introduces the characteristics of the ballast water system for the large semi-submersible vessels, and the difficulties and solutions for the ballast water treatment for large semi-submersible vessels. By comparing the ballast treatment system for the typical large semi-submersible vessels, it is concluded that the circulation method and the gravity method are reasonable and feasible solutions for the ballast water treatment system of the large semi-submersible vessels. It can provide reference for the ballast water treatment system of the large semi-submersible vessels.