Large-scale Mg-8Gd-4Y-1Zn-Mn (wt.%) alloy ingot with a diameter of 315 mm and a length of 2410 mm was prepared through semi-continuous casting. Chemical composition, microstructure and mechanical properties at different locations of the samples with as-cast, T4 and T6 heat-treated states, respectively, were investigated. No obvious macro segregation has been detected in the high-quality alloy ingot. The main eutectic structures at all different locations are composed of α-Mg, Mg₃RE-type, Mg₅RE-type and LPSO phases. At the edge of ingot, the unusual casting twins including {101-2} extension twins and {101-1} compression twins were observed due to the intensive internal stress. In T4 heat-treated alloy, the micro segregation was eliminated. The remained phases were α-Mg and LPSO phase. Combined with the remarkable age-hardening response, T6 samples exhibits improved mechanical properties at ambient temperature, which derives from the dense prismatic β' precipitates and profuse basal γ' precipitates.