Abstract: In the semi-continuous casting process of 5005 aluminum alloy flat ingot with cross-section size of 1 330 mm×460 mm. The aluminum alloy melt in distribution launder was treated by pulsed magnetic field with duty cycles of 40%, 50% and 60% with applying self-developed pulsed magnetic field melt treatment device. The effect of pulsed magnetic duty cycle on solidification structure and element segregation of 5005 aluminum alloy flat ingot was studied. The results show that the solidification structure of 5005 aluminum alloy flat ingot could be refined significantly by applying pulsed magnetic field on melt. With the increase of pulsed magnetic field duty cycle, the α-Al grain thinning effect was weakened. When the duty cycle of pulse magnetic field was 40%, the α-Al grains at different positions of the flat ingot cross-section were the smallest, and the α-Al grains at the center were refined by 24.8% with those without pulsed magnetic field treatment. With the increase of the pulsed magnetic duty cycle, the microstructure uniformity of the flat ingot decreased. When the pulsed magnetic duty cycle was 40%, the microstructure uniformity of the flat ingot was the highest, and the difference of α-Al grain size between the thickness edge and the center of the ingot was 6.6%. After the pulsed magnetic field melt treatment with 40% duty cycle, the macrosegregation of alloying elements in the flat ingot was reduced.