Abstract In this paper, the effect of semi-solid isothermal heat treatment on the microstructure of Mg–6A1–1Zn–0.7Si alloy, especially on the semi-solid microstructural evolution and the modification of Chinese script shaped Mg 2Si phase, are first investigated. The research results indicate that it is possible to produce the Mg–6A1–1Zn–0.7Si alloy with non-dendritic microstructure by the semi-solid isothermal heat treatment. After treated at 575–585 °C for 120 min, the experimental alloy can obtain a non-dendritic microstructure with a 12–21% liquid content and an average size range of 48–67 μm of the unmelted primary solid particles. In addition, the semi-solid isothermal heat treatment can modify the Chinese script shaped Mg 2Si phases in experimental alloy. After treated at 580 °C or 585 °C for 120 min, the morphology of Mg 2Si phases in the experimental alloy changes from the initial Chinese script shape to granule and/or polygon shapes.