Abstract: The microstructure and properties of twin-roll continuous casting Al-Mg-Mn alloy slab were studied by optical microscopy(OM), scanning electron microscopy(SEM), differential scanning calorimetry(DSC), electrical conductivity and hardness test. The optimization mechanism of twin-roll continuous casting process was analyzed. The results show that melt in semi-solidified state is strongly affected by cooling water. Heat is transferred forward from the back end of the cast-rolling zone, and the initial dendrite grows in the direction opposite to the heat flow, and the growth direction near the center is almost horizontal. The shape of the sump changes greatly, which effectively improves the macro-segregation defect. With the increase of slab thickness, the shape of sump changes more significantly, and the cooling rate decreases gradually, and the alloying elements diffuse fully. When the thickness reaches 20 mm, the cooling rate is the slowest, and the defects of macrosegregation and intergranular segregation basically disappear. The alloying element distribution is the most diffuse, the solid solution has the highest supersaturation, and the alloy has the highest hardness and the lowest conductivity, which are 92.26HV and 17.74 MS/m, respectively.