An online antifouling ship ballast water treatment system comprises a ballast water main pipeline (11), a ballast water branch pipeline (18), a chemicals feeding pipeline (13), a filter (2) located on the ballast water main pipeline (11), an electrolysis unit (3) located on the ballast water branch pipeline (18), and a chemicals feeding pump (12) located on the chemicals feeding pipeline (13). Part of seawater introduced into the ship ballast water treatment system is filtered by the filter (2), then flows to the ballast water branch pipeline (18) and the electrolysis unit (3), the part of the seawater electrolyzed by the electrolysis unit (3) is reinjected to the filter (2) through the chemicals feeding pump (12) and the chemicals feeding pipeline (13) after a ballast process is completed, and the part of the seawater is kept in the filter (2) for a certain time. After the ballast process is completed, a TRO solution obtained by electrolysis treatment of the seawater is reinjected to the filter (2) and is ke
