The　mechanical　properties　and　microstructure　of　Al-4.5Mg-0.7Mn-0.4Er　alloy　prepared　with　iron　mold,　water　cooling　mold　and　semi-continuous　casting　have　been　described　by　tensile　testing,　optical　microscope,　SEM　(scanning　electronic　microscope),　XRD　(X-ray　diffraction)　and　TEM　(transmission　electron　microscope).　It　is　found　that　increasing　cooling　rate　can　improve　the　strength　of　Al-4.5Mg-0.7Mn-0.4Er　alloy,　and　be　helpful　for　the　formation　of　supersaturation　solid　solubility　of　Er　and　Mn　element　in　Al　during　solidification　process　and　for　the　precipitation　of　fine　and　dispersive　Al3　Er　phase　and　Al6　Mn　phase　after　homogenized　annealing　treatment.　Fine　and　dispersive　Al3　Er　phase　can　be　acted　as　the　effectively　strengthening　phase　in　the　Al　alloy,　which　is　helpful　for　improving　the　mechanical　properties　and　microstructure　of　the　alloy.