In the 10th year of Dazhong (856 CE), Tang court sent emissaries to the Western Uighur kingdom in the eastern Tianshan, hoping to erect a new Kaghan, but the emissaries were robbed by “betrayed Uighurs” and lost all of the materials for the Kaghan. This is an important event in the history of the northwestern regions of China during the late Tang. This author studies in detail the complete picture of the event, based on traditional sources, Dunhuang manuscript Zhang Yichao bianwen, and the newly discovered epitaph of Li Xun, who was the vice messenger of the emissaries.