Emperor Taishang (太上皇) is a title given to the surviving father of an abdicated emperor or a current emperor in Chinese history, usually given to an emperor who is alive but has ascended the throne.
Although some Emperor Taishang was already in Zen, he still had real power in his hands, such as Emperor Xuan of Northern Zhou, Emperor Gaozong of Song, Emperor Gaozong of Qing, and Emperor Xianwen of Northern Wei; most of the Emperor Taishang was actually forced by the situation rather than voluntarily abdicated, such as Tang Xuanzong, Song Guangzong, etc., among which Tang Gaozu Li Yuan was forced to cede the throne to his second son Li Shimin after the "Xuanwumen Revolution", and this history can be said to be well known.
Li Shimin is the famous Ming Emperor and Renjun Tang Taizong in Chinese history, who changed his name to Yuan Zhenguan after he ascended the throne. He ruled the world with civility, humbly accepted advice, strictly practiced economy, and created the famous "rule of Zhenguan" in Chinese history. After his abdication, although Li Yuan was honored as emperor Taishang, his life was extremely depressed, mainly because he was often counted down by his son Li Shimin and was picked up for various problems.
As soon as Li Shimin came to power, he directly turned the gun on his father and criticized various maladministrations during the Wude years. In the courtroom, he asked the minister, "Is it beneficial to the whole world to seal the sect' sons?" "If you are the Son of Heaven, so you can raise the people, how can you work for the people to feed themselves?" Obviously, it means that Li Yuan should not divide the seals on a large scale, and as the Son of Heaven, he should support the people instead of letting the people provide for these clans. Subsequently, he relegated all the clan counties that Li Yuan had once divided into county princes, and the courtiers understood Li Shimin's meaning and all echoed them. Such a demeaning of his father and his own behavior did make Li Shimin win the hearts and reputation of the people, but it made Tang Gaozu Li Yuan discredited and embarrassed.
Li Shimin also gradually purged the political forces around Li Yuan, Pei Huan had been appointed by Li Yuan as the prime minister, but as soon as Li Shimin came to power, he first found a reason to exile him, Pei Huan was not convinced, Li Shimin said to him: "Ji Gongxun Yong, not to this, in vain, with grace, especially the first." At the time of Wu De, the political punishment was fierce, the official was in order, and the reason for the office was public. But with old feelings, we can't do the utmost law, and we can sweep the grave, so why should we resign? "The main meaning is that if your merits are obviously unworthy of virtue and merit, but it is only because of the relationship with the Emperor Taishang that you are the prime minister, during the Wude years, the political punishments are false, and the common way that officials should abide by has been messed up and destroyed, and these should be attributed to the responsibility of your prime minister Pei Ning." Now that I remember my old feelings, I will not punish you with capital punishment and let you go back to your hometown to sacrifice and sweep the grave, so why should I resign?
In addition, Li Yuan's personal freedom was also restricted, and after Li Shimin ascended the throne, he moved Li Yuan from the comfortable Taiji Palace to the Da'an Palace (i.e., Hongyi Palace), which actually put him under house arrest.
After Li Yuan abdicated, although he had the lofty status of emperor Taishang and lived a carefree life, he was really depressed, after all, Li Shimin had obtained the throne through a coup d'état, and it was inevitable that he would be wary of Li Yuan, and the expulsion of Pei Yuan was probably a fear that Li Yuan would carry out restoration under Pei Yuan's planning.