With the growing demand for rapid and cost-effective solutions for lightweight magnesium alloys with excellent high-temperature mechanical properties, we investigated a heat-treatable RE-free magnesium alloy, Mg-4Sn-3Al-1Zn (TAZ431, wt.%), with remarkable thermal stability. The peak-aged TAZ431 alloy exhibits a 15 % increase in high-temperature (230 ℃) yield strength, compared to the traditional commercial magnesium alloy, Mg-3Al-1Zn (AZ31, wt.%). In this work, we demonstrate that the basal Mg₂Sn precipitates have a more pronounced hindrance effect on the non-basal slip systems, as evidenced by a combination of experiments (in-situ EBSD and dual beam TEM) and numerical simulations (Orowan model and VPSC). The distribution and morphology of Mg₂Sn precipitates and cracks are quantitatively analyzed using a range of techniques including in-situ SEM and synchrotron X-ray tomography. Our results reveal that the decohension of grain boundary precipitates significantly promotes the formation of intergranular cracks, leading to ultimate fracture. The research comprehensively explains the impact of particle morphology, orientation, and distribution on precipitation strengthening and fracture modes at elevated temperatures, which is vital for the future development of high-temperature performance magnesium alloys.