Abstract: The effect of different Si content on the microstructure and properties of heat treatment free die casting Al-Si alloy was studied. The results show that the precipitates in Al-6% Si alloy mainly precipitated along the grain boundary and distributed unevenly,which shape is like a rod,with sharp edges and corners.With the increase of Si content,the proportion of precipitated phase increases significantly,and the particle size increases. At the same time,there is a small amount of undissolved massive primary silicon.In Al-Si alloy,there are mainly two different types of precipitates,one is long needle like AlSiFe phase,the other is bright coarse block like AlSiMnFe phase. The long needle like AlSiFe phase has high surface energy and owns enough vacancies to absorb Mn atoms,which can spontaneously transform into a more stable blocky AlSiMnFe phase during solidification process. In this process,Mn element is largely consumed,the Mn/Fe value in precipitated phase particles decreases. With the increase of Si content,the tensile strength of the sample is significantly improved,but the plasticity and impact resistance are not improved together.