Emperor Taizong Receiving the Tibetan Envoy, a hand-scroll traditionally attributed to Yan Liben(601-673), is a contemporary visual documentation of the diplomatic encounter between Tang and the kingdom of Tibet,immediately after the later was defeated and accepted tributary status in 641 AD. In 833, nearly two centuries after the encounter, Li Deyu(787-849), the chief minister of the court, ordered the hand-scroll to be remounted and wrote a postscript at the end of it, which recounted the encounter in a new perspective. Previous studies have mostly focused on whether the painting is an authentic work of the Tang Dynasty or a copy of the Song Dynasty, who the author is, what the specific historical scenes it presents, and its collection history, but few scholars have discussed the motivation, function and influence of Li Deyu's remounting and postscript. This paper provides new evidence that the original postscript was written by Li Deyu from two aspects:the format of the postscript and the book of Prime Tortoise of the Record Bureau; Through the analysis of the background and new postscript, it reflects a border incident between Tang and the kingdom of Tibet that Xidamou who was the defender of Weizhou City, a general of the kingdom of Tibet, surrendering to the Tang Dynasty. Due to the revenge of Niu Sengru, Li Zongmin and others, Li Deyu experienced the failure of politics and morality in this event. The injustice or humiliation he suffered was the most unforgettable memory in his political career, so he was always looking for opportunities to plead and defend himself. Two years later, when he came to power, he met the sorting and collection activity of calligraphy and painting in the court. At this time, he did not recover from the new injury or expresse his hatred yet, so, he presided over the remounting of the Emperor Taizong Receiving the Tibetan Envoy and did not tire of making postscripts. The postscripts are not a memory of the Taizong meeting with Lu Dongzan about 200 years ago, but an expression of the political humiliation he suffered two years ago; meanwhile, the postscripts also indicated Li Deyu's experience when he served as the Jiannan Xichuan Jiedushi, and the frontier policy he advocated. To be specific, the content of his postscripts may not only cause his pain for his fate, but also give historical legitimacy to his frontier policy. The postscripts reconstructed the events shown in the Emperor Taizong Receiving the Tibetan Envoy seems to imply that his handling of the surrender of Weizhou followed the principles and strategies of Taizong; However, emperor Wenzong, Niu Sengru and Li Zongmin betrayed those who surrendered and caused them to be tortured and killed, which violated the strategy how the empire controlled tributary states established by Taizong. In other words, Li Deyu's postscripts to the Taizong Receiving the Tibetan Envoy took advantage of the past events in the Taizong Dynasty and the policy to the kingdom of Tibet as a metaphor, secretly satirized the surrender of Weizhou city, and defend himself. A series of events, such as remounting the scroll, writing down postscripts, and asking emperor Wuzong to grant Xidamou an official post, not only relieved the pain of conscience and moral torture suffered in the past 12 years, but also revealed the different opinions of the Niu and Li parties on the policy to the kingdom of Tibet at that time. To a certain extent, it also reflected the position and strategy of the mid and late Tang court on the separatist regimes of Uyghurs, Xiajiasi, and other frontier nationalities. This paper is an art history case to explore how the ancients used calligraphy and painting collections as well as inscriptions as political agency to implicitly convey their administrative strategies.