The utility model belongs to the technical field of primary side maintenance equipment for steam generators of pressurized water reactor nuclear power plants, and in particular to a high water blocking board of a primary side main connecting pipe of a steam generator. The high water blocking board comprises two side boards and a center board, wherein the center board is installed between the two side boards, and the center board and the two side boards are matched to form a full circular structure. Each side board is equipped with a detecting assembly, and all the center board and the two side boards are embedded in a sealing assembly. The side boards are fixedly installed on the primary side main connecting pipe through double-headed screw assemblies, and the double-headed screw assemblies are installed on the main connecting pipe through bosses. The center board is fixedly installed on the main connecting pipe through a locating pin. The blocking board can be used for sealing a primary side water chamber of the steam generator before high water arrives to ensure the required time of the maintenance process.