Abstract: The Al-Si-Mg alloy is a kind of casting aluminum alloy that can be strengthened by heat treatments. Second dendrite arm spacing (SDAS) has great influence on the mechanical properties of alloy. Therefore research on the influences of SDAS on aging kinetics of A357 alloy has great significance to increase its performances. Thispaper studies the influences on aging kinetics of A357 Alloy with hardness tester and metallographic analysis with optical microscope. The results show that, under the same solution conditions, the hardness after solution treatment reduces with the SDAS's increasing; the ageing peaks lag with the increasing of SDAS. Regression analysis and dimensional analysis are employed to establish an equation, whose rationality is testified byfollowing analysis．