High-pressure die-casting (HPDC) was used to develop two sizes of Al-7Si-0.2Mg (mass fraction, %) thin-plates with similar composition. Thin-plate castings with varying degrees of porosities and uneven microstructures were obtained and their effects on ductility were investigated. The results showed that the elongations of specimens from different castings and different positions fluctuated greatly (9.7%-17.9%). Elongation dropped sharply when specimens contained large porosities, which could be ascribed to stress concentration due to the reduction of the effective bearing area. However, for samples with small porosities, α-Al(Fe/Mn)Si phase cracked prior to eutectic phase during plastic deformation. The number density of α-Al(Fe/Mn)Si phase played an important role in the fluctuation of elongation, and specimens with higher number density of α-Al(Fe/Mn)Si particles exhibited deteriorated elongations. In addition, the increase in the number density of α-Al(Fe/Mn)Si phase was attributed to local higher cooling rates.