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The effect of Cr content and cooling rate on the microstructure of Al-Mn alloy was studied using well resistance furnace melting, and the alloy was analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The experimental results showed that adding Cr could significantly improve the morphology of the primary phase in the Al-2.5Mn alloy. Without Cr, the primary phase in the alloy was thick, needle-like, and strip-like structure. After adding 0.2wt%-0.5wt% Cr, the primary phase in the upper part of the alloy was gradually fined and reached the best effect at 0.35wt% Cr. When the content of Cr was 0.5wt%, the microstructure of the primary phase in the upper part began to coarsen. The bottom of the alloy was a large bulk phase, but still much finer than that without adding Cr. XRD and SEM analysis showed that the precipitation phase at the bottom was mainly Al85Mn7Cr8, while the fine microstructure at the top was Al6Mn and Al3Mn. The results of the cooling rate experiments showed that the primary phase of Al-2.5Mn-0.35Cr was further refined, and the eu-tectic microstructure was partly achieved, under air-cooling condition. And when the cooling method was iron die-cooling, the microstructure of the Al-2.5Mn-0.35Cr alloy was changed into a eutectic microstructure.