The invention discloses a high-throughout continuous casting and rolling Al—Mg—Mn alloy plate for ships and the preparation process thereof. The chemical components of the Al—Mg—Mn alloy in percentage by mass percentage are: Mg: 0.80-2.80%, Mn: 0.00-1.40%, Zr: 0.10-0.50%, Cr: 0.15-0.35%, Sr: 0.00-0.10%, Er: 0.00-0.60%, Si: 0.10-0.40%, Cu: 0.01-0.10%, Ti: 0.01-0.05%, Fe: 0.00-0.40% and the rest is Al. The preparation processes mainly include smelting and melt treatment, continuous casting, continuous rolling and cold rolling. The invention solves the problems of easy segregation, low strength and toughness and poor formability in the preparation of high-throughout continuous casting and rolling Al—Mg—Mn plates for ships.