Semi-solid isothermal heat treatment microstructural evolution of die casting Al-5Mg-2Si-Mn alloy with different injecting pressure was studied. The average α-Al grain size of as-cast die casting experimental alloys was smaller than that of gravity casting, and it decreased while microstructure distortion energy increased with the increase of the injecting pressure. It was found that semi-solid microstructural evolution process of die-casting alloy was similar with, but faster than that of gravity casting. The results from regression analysis show that the relationship between the average α-Al grain diameter and the isothermal holding time fits the formula r ?^3=Kt+〖r ?_0〗^3, and the coarsening rate (K) is decreased with the increase of the injecting pressure. For the fixed injecting pressure, the average α-Al grain size of the experimental alloy increased while the K reduced with the increase of the isothermal holding temperature. For the fixed isothermal holding time, the solid fraction and α-Al grain coarsening rate of die-casting alloy were lower than those of gravity. The appropriate semi-solid isothermal heat treatment parameters of the die casting experimental alloy were determined to be 620 ℃ for 20 min.