Radial forging was introduced to the strain-induced step in the strain-induced melt activation process to prepare high-quality semi-solid A356.2 billet for the high solid fraction compression. Then, the deformation behaviour and microstructures at different compression velocities, temperatures and deformation zones were investigated. The results showed that radial forging can induce enough strain at 60% reduction of area to prepare ideal semi-solid microstructure. The microstructure had no obvious improvement at 75% reduction of area because the distortion energy may be saturated at 60%. During compression tests, the flow stress was sensitive to compression velocity (V_c) but was insensitive to holding temperature (T_h), and it obeyed the power law p=Kc·{macron}m. The average strain rate sensitivity was 0.2757, and the average apparent activation energy was 254.5 kJ/mol. The compression sample can be divided into hard deformation zone, transition deformation zone, severe deformation zone and free deformation zone. Different zones had different predominant deformation mechanisms which not only affected the morphology of the grains but also codetermined the flow stress. With increasing V_c and T_h, the average particle size was increased from hard deformation zone, reached the maximum at severe deformation zone and then decreased at free deformation zone, while the opposite was the case for shape factor. At each deformation zone, the average particle size was increased while the shape factor was decreased with the increasing V_c because higher V_c caused more grains to merge into larger and more irregular grains, while the opposite was the case for the increasing T_h because higher T_h generated more liquid which helped to isolate the contacted particles and improve their spheroidization degree.