The Al-12%Si alloy powder with particle size of 50～100 μm was printed on 6063 aluminum alloy substrate by powder feeding laser melting 3D printing device. The microstructure evolution of 3D printing Al-12%Si alloy with/without transverse steady magnetic field was studied. The results show that the deposited samples obtained by laser melting show obvious laminar accumulation characteristics, and the composition contains a bright white α-Al phase and an Al-Si eutectic phase and a part of the darker α-Al phase, and the sample density reaches 94%. At the magnetic field strength of 0.5 T, the bead interface is formed in the bottom region of the sample and a plurality of equiaxed α-Al phases are formed at the edges. The magnetic field has the effect of converting the columnar crystals into equiaxed crystals. Moreover, the α-Al phase can be converted from a columnar crystal to an equiaxed crystal, and a high-order dendritic arm can be formed.