Abstract: An experimental inclined cooling/shearing set-up was developed to prepare the Al-Mg-based semisolid alloy, the molten alloy nucleated heterogeneously on the inclined-cooling plate surface and was sheared under the action of gravity force, and its microstructure gradually evolved into fine spherical grain from coarse equiaxed grain network. By controlling casting temperature in the range of 660-690°C, the semisolid slabs with good microstructure were obtained, with the feather crystal which are usually found the high-Mg alloy slabs conventionally improved greatly. The surface conditions of cooling plate obviously affects the alloy microstructure, i.e., fine microstructure can be obtained if the cooling plate has a clean surface that is rough to a certain degree. The inclination angle of cooling plate is preferable in the range of 40°-50°.