Abstract: The as-cast Al-12.0Si-4.0Cu-0.7Mg-1.5Ni alloy was treated by two-stage solution+aging.The effects of the second stage solution temperature and solution time in two-stage solution on the microstructure and hardness of the as-cast Al-12.0Si-4.0Cu-0.7Mg-1.5Ni alloy were studied by SEM, XRD and hardness test.The results show that the microstructure of as-cast Al-12.0Si-4.0Cu-0.7Mg-1.5Ni alloy is mainly composed of α-Al solid solution, massive primary Si phase, acicular eutectic Si phase and reticular second phase.After two-stage solution treatment, the massive primary Si phase in the alloy is passivated obviously, the eutectic Si phase is refined obviously, and the reticular second phase is also significantly fragmented.At 490℃×2 h+525℃×4 h, the reticular phase is almost completely fragmented, and the eutectic Si phase is the smallest.With the increase of the second stage solution temperature and solution time in two-stage solution, the hardness of T6 Al-12.0Si-4.0Cu-0.7Mg-1.5Ni alloy increases first and then decreases.The hardness of the alloy treated by 490℃×2 h+525℃×4 h double-stage solution+180 ℃×6 h aging is the highest, reaching 88.2 HRB.