In 762, both Emperor Xuanzong (then
Taishang Huang (retired emperor)) and Emperor Suzong died in rapid succession, and Emperor Suzong's son
Emperor Daizong took the throne. Du Hongjian was put in charge of the burials of Emperors Xuanzong and Suzong. After the two emperors were buried, he was given the honorific title of
Guanglu Daifu () and created the Duke of Wei. In 764, Emperor Daizong made him the deputy minister of defense again (now with the title
Bingbu Shilang () and gave him the designation
Tong Zhongshu Menxia Pingzhangshi (), making him a chancellor
de facto. He was soon also made
Zhongshu Shilang (), the deputy head of the legislative bureau. In 765, when Emperor Daizong entered into a peace treaty with
Tufan, he had Du and fellow chancellor
Yuan Zai sign the treaty with the Tufan emissary. In 766, after Xichuan Circuit (西川, headquartered in modern Chengdu) was thrown into a state of civil war after the military governor Guo Ying'ai () was killed by the military officer
Cui Gan and several other military officers—Bo Maolin (), Yang Zilin (), and Li Changkui ()—rose to resist Cui, Emperor Daizong made Du the military governor of Xichuan as well as the deputy supreme commander of the region, to try to calm the circuit. On the way, though, Du heard that the military governor of Shannan West Circuit (山南西道, headquartered in modern
Hanzhong,
Shaanxi), Zhang Xiancheng (), had been defeated by Cui, and was afraid to advance. He sent messengers to make overtures to Cui, who acted as respectfully as possible and offered bribes to Du. When Du arrived at Chengdu, Cui treated him with the utmost formal respect but did not permit him any actual control over the governance. In response, Du repeatedly recommended to Emperor Daizong to let Cui become military governor while placating Bo, Yang, and Li Changkui by making them prefects of their prefectures. Emperor Daizong reluctantly agreed, although he initially made Cui only the mayor of Chengdu Municipality and the military commander, under Du. In 767, Du requested to return to Chang'an. Emperor Daizong agreed, and he made Cui the military governor and let Du resume his service as chancellor and made Cui the military governor—an action for which the people of the time criticized Du for, believing that he was encouraging rebellion. Upon his return to Chang'an, Du, a devout
Buddhist, offered thanks to the
Buddha by holding a vegetarian feast for over 1,000
Buddhist monks. Meanwhile, it was said that because Du and his chancellor colleagues Yuan and
Wang Jin were all devout Buddhists, Emperor Daizong and the other officials were all influenced by them to become devout Buddhists as well. In 769, Emperor Daizong wanted to put Du in charge of Luoyang, to replace Wang. Du declined, citing an illness, and was allowed not to go to Luoyang. He further resigned his other posts, including chancellor post, late in 769，which was approved on 10 December; he died three days later. Prior to his death, he undertook
tonsure and formally became a Buddhist monk, and by his will was cremated with his ashes placed in a tower, pursuant to
Buddhist customs, rather than buried in the ground per Chinese customs of the time, and was much criticized by popular opinions of the time. He was awarded posthumous honors.
Liu Xu, the lead editor of the
Old Book of Tang, commented about Du: == Notes ==