U-shaped thin and high arch dam is rarely seen all over the world. Qinglong arch dam in Hubei province is especially significant on account of its special terrain conditions. It is an RCC thin arch dam constructed in particular U-shaped river valley. The stress distribution of Qinglong arch dam is significantly different from that of conventional V-shaped dam. A large tensile stress area was found in the downstream arch crown which is 1/4 dam height from the bottom of dam. In view of this, discussions on the failure mechanism and safety of the dam are imperative. On the basis of the Drucker-Prager criterion, the ultimate bearing process of Qinglong arch dam is calculated and analyzed by using strength accumulation coefficient and overloading coefficient. Furthermore, by recording the abrupt change of dam displacement and observing the connected plastic zones and the convergence in the evolutionary process, the failure mechanisms of Qinglong arch dam is discussed and the safety degree is evaluated. Results suggest that different simulation methods lead to different destruction modes of dam in the upstream and downstream. But the final destruction patterns are consistent in general. As the stress of Qinglong arch dam in the ultimate bearing process adjusts itself, the tension stress decreases or disappears, and the safety of dam meets stability requirements. In addition, the characteristics of progressive failure mode of the dam are summarized. The research results offer important reference for similar projects.