A stepwise-stacked seawater battery assembly comprises a first battery chamber, a second battery chamber, a seawater electrolytic liquid, and an electric-conduction set. The first battery chamber includes a first accommodation room and a first electrode group. The second battery chamber is disposed above the first battery chamber and includes an opening, a second accommodation room interconnecting with the opening, and a second electrode group disposed in the second accommodation room. The seawater electrolytic liquid includes a first electrolytic liquid received by the first accommodation room and a second electrolytic liquid received by the second accommodation room. The electric-conduction set electrically connects the first electrode group and the second electrode group. The structural design that the first battery chamber receives the first electrode group and the second battery chamber receives the second electrode group can simplify the structure of the seawater battery assembly.