He was credited with reforming the tax system to reduce burdens on the peasants and to bring merchants into the rank of taxpayers, but was blamed for using his position to take vengeance on political enemies.
[1] His great-grandfather Yang Dabao (楊大寶) served as a county magistrate during the reign of the Tang dynasty's founder, Emperor Gaozu, and was killed when resisting a rival contender for the throne, Liu Wuzhou the Dingyang Khan, and therefore was posthumously honored.
[2] Yang Yan was known in his youth for his handsome eyebrows and beard, his ability to stand up against monetary enticements, and his beautiful writing.
Later, he took off his hermit robes and served as a secretary under Lü Chongbi (呂崇賁) the military governor (Jiedushi) of Hexi Circuit (河西, headquartered in modern Wuwei, Gansu).
When he wrote the text of a monument dedicated to the general Li Kailuo (李楷洛), the writing was so beautiful that the scholars at the time all took pains to memorize it.
He later became the deputy minister of civil service affairs (吏部侍郎, Libu Shilang) and was put in charge of editing the imperial history.
[3] Emperor Dezong, wanting to reform government, was willing to promote people in exceptional manners, and in fall 779, based on the recommendation of the chancellor Cui Youfu, Emperor Dezong promoted Yang from exile to be the Menxia Shilang (門下侍郎), the deputy head of the examination bureau of government (門下省, Menxia Sheng) and gave him the designation Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事), making him a chancellor de facto, serving alongside Cui and Qiao Lin.
During the Zhide era [(756–758, during Emperor Suzong's reign)], the state was in war and chaos, and the collections of taxes were both done in an emergency and a haphazard manner.
Still, at Yang's urging, he removed Liu from his various posts as directors of specialized financial agencies and returned the responsibilities to the executive bureau (尚書省, Shangshu Sheng).
[4] By spring 780, With Cui Youfu seriously ill at that point and Qiao having been removed by Emperor Dezong due to incompetence, Yang was solely in charge of the government, and he revived major military projects that Yuan had advocated until his downfall—to rebuild Yuan Prefecture (原州, in modern Guyuan, Ningxia), formerly Tang territory but which had become part of the no man's land between territories held by Tang and Tufan, as a forward advance base for a campaign to recapture lands lost to Tufan; and to conscript labor from the regions of the two capitals (Chang'an and Luoyang) and Guanzhong to reopen Lingyang Aqueduct (陵陽渠, flowing through Bayan Nur, Inner Mongolia) in order to promote agriculture in the region by soldiers.
[4] In fall 780, at Yang's direction, Liu Yan's superior Yu Zhun (庾準) the military governor of Jingnan Circuit (荊南, headquartered in modern Jingzhou, Hubei), accused Liu of complaining about Emperor Dezong in a letter to the general Zhu Ci, and of preparing to commit treason.
The warlord Li Zhengji, the military governor of Pinglu Circuit (平盧, headquartered in modern Tai'an, Shandong), in particular, was submitting repeated petitions criticizing the imperial government for executing Liu.
Li Na and Li Weiyue, along with their allies Tian Yue the military governor of Weibo Circuit (魏博, headquartered in modern Handan, Hebei) and Liang Chongyi the military governor of Shannan East Circuit (山南東道, headquartered in modern Xiangfan, Hubei), thereafter prepared for war against the imperial government.
Emperor Dezong agreed, and in fall 781, he made Yang Zuo Pushe (左僕射), one of the heads of the executive bureau, but no longer chancellor.
At Lu's direction, Yan Ying soon exposed an incident in which Yang had asked Zhao Huibo (趙惠伯) the mayor of Henan Municipality (河南, i.e., the Luoyang region) to sell Yang's mansion for him—and, at Yan's manipulation, this incident was increased in severity from the lesser charge of conflict of interest to embezzlement.
In winter 781, Emperor Dezong demoted and exiled Yang to the post of military advisor to the prefect of Yai Prefecture (崖州, in modern Sanya, Hainan).