Earth-rock dams may encounter high-intensity earthquakes during service, which poses a threat to the safety of post-earthquake operation. Hence, based on the actual monitoring data, XGBoost algorithm is used to invert the material parameters of a high gravel clay core wall dam that has experienced an earthquake during its service period. Based on the inversion result, we adopted the seismic time-history method to evaluate the seismic performance and the ultimate seismic capacity of the dam, with the dam crest settlement rate as the evaluation index. The results show that the material strength of the dam is still in a good state after the earthquake, compared with the design parameters. When the peak seismic acceleration is 0.55g, the dam crest settlement rate reaches 1.3%; therefore, the ultimate seismic capacity of the gravel clay core wall dam is 0.55g. When the shock resistance lasts for 30 seconds, there is a risk of hydraulic fracturing in the core wall at the elevation above 820 m, and the main stress curve near 700 m tends to be slightly close to the water pressure line, which is also an area prone to hydraulic fracturing.