Abstract: The high-temperature service environment in the aerospace field may cause thermal damage to rare-earth magnesium alloys, which leads to the deterioration of structural properties. The detection and characterization of rare-earth magnesium alloys with high-temperature thermal damage is of great significance to ensure the safety of equipment operation. Thermal aging tests were carried out at a temperature of 200°C to obtain specimens with different degrees of thermal damage. The nonlinear ultrasonic measurement experiment was performed using the selected Lamb wave S1-S2 mode pair with accumulation effect according to the nonlinear Lamb wave resonance conditions. Combined with the microstructure observation of heat-damaged specimens using transmission electron microscopy, the microscopic mechanism of nonlinear parameter change was analyzed quantitatively. The results show that the normalized nonlinear parameter increases monotonically with the thermal damage of magnesium alloy, and the nonlinear parameter is 151.5% of the initial value when the thermal aging time is 192 h. The thermal aging precipitation sequence of VW63Z magnesium alloy is β'' → β' → β, where the three precipitates alternately precipitate. During this period, structural changes and size coarsening lead to lattice mismatch between the precipitated phases and the matrix phase, resulting in accumulative growth of the normalized nonlinear parameter with the thermal aging time. The research shows that nonlinear ultrasonics can effectively characterize high-temperature thermal damage of magnesium alloys.