The invention relates to a method for rolling a magnesium alloy sheet used as the anode of a sea-water activated battery, which is characterized in that smelted heavy metal element-containing high-temperature magnesium alloy liquid is transported and distributed by a heat-preservation liquid-supplying nozzle to flow into the roll gap of the internal water cooled rolls of a casting and rolling machine, temperature difference between the high-temperature magnesium alloy liquid and the internal water cooled rolls is kept to solidify the magnesium alloy liquid in 1-4 s, plastic deformation is immediately carried out so that a fine grain microstructure strip with out macrosegregation is formed, wherein, the depth of the strip is less than 4 mm and the size of the fine grain is 30-10 um, then, stretching force is applied to the cast-rolling plate strip plate which is evenly heat treated under the ramollescence of high-energy electrical pulse so as to carry out cold rolling and fine finish into the magnesium alloy sheet with high precision.