Cast Al-Si alloy is widely used in cylinder head structures and long-term service in 200–250 °C elevated temperature environments, which is easy to cause the deterioration of alloy properties. In this study, the thermal stability of the alloy during thermal exposure at 200 °C and 250 °C is investigated through the microstructure characterization and the mechanical properties test. The results show that the thermal exposure leads to the damage evolution of the precipitates in the microstructure. With the coarsening of the precipitates, the dislocation changes from cutting precipitates to bypassing precipitates, resulting in a weakening of the precipitation strengthening effect. The T6 alloy is in the insufficient peak aging state, and the alloy first reaches the peak aging state and then enters the over aging stage during the thermal exposure. Overall, with the increase in thermal exposure time and temperature, the Al matrix hardness, ultimate tensile strength (UTS), and yield strength (YS) decrease gradually and finally tend to be stable. As the thermal exposure progresses, the alloy fracture characteristic transitions from brittle fracture to ductile fracture, and the elongation of the alloy increases. In addition, the coarsening process of the thermal exposure alloy is described based on the Lifshitz-Slyozov-Wagner (LSW) theory, and the modified coarsening kinetic model can be divided into the rapid roughening stage and the stable roughening stage. Further, the damage parameter is defined as the relative change of precipitation strengthening to describe the deterioration degree of the mechanical properties for thermal exposure alloy. Display omitted •The thermal exposure results in a weakening of precipitation strengthening effect.•The alloy through the T6 heat treatment is in the insufficient peak aging state.•Alloy mechanical properties degrade with an increase in exposure time and temperature.•The coarsening process of precipitates is divided into rapid stage and stable stage.•The decline degree of the mechanical property is described by the damage parameter.