The Palace of Heavenly Purity is the main hall among the three palaces in the inner court and is the exclusive sleeping palace for the emperor. It was built in the early Yongle period of the Ming Dynasty and the current building was rebuilt in the third year of Jiaqing period of the Qing Dynasty. "Qianqing" means "pure and upright".

The fourteen emperors of the Ming Dynasty and the two emperors Shunzhi and Kangxi of the early Qing Dynasty all used the Qianqing Palace as their sleeping palace. They lived here and handled daily government affairs here. During the Shunzhi and Kangxi periods of the Qing Dynasty, the Qianqing Palace had a close relationship with government affairs. The two emperors studied, reviewed memorials, summoned officials, received foreign envoys, and held inner court ceremonies and family banquets here. Although Emperor Yongzheng moved to the Yangxin Palace, he still often came here to select officials and review memorials. Emperors Kangxi and Qianlong of the Qing Dynasty also held a banquet for thousands of elders in the Qianqing Palace. After the death of the emperors of the Ming and Qing Dynasties, a mourning hall was set up here to show that they had "died of old age."

Inside the Palace of Heavenly Purity, there hangs a plaque inscribed with the words "正大光明" written by Emperor Shunzhi of the Qing Dynasty. Behind this plaque is a "储箱" for secretly establishing a crown prince. At that time, the struggles between the princes for the throne were fierce. In order to ease this contradiction, the secret establishment of a crown prince was adopted since the reign of Emperor Yongzheng. The emperor did not publicly establish a crown prince during his lifetime. Instead, he placed one copy of the document for establishing the successor to the throne beside him, and another copy was sealed in the "储箱" and placed behind the "正大光明" plaque. After the emperor's death, the ministers of state jointly took down the "储箱" and compared it with the copy that the emperor kept secretly beside him. After verification, the successor to the throne was announced. Emperors Qianlong, Jiaqing, Daoguang, and Xianfeng all ascended the throne according to this system. There is a throne in the center of the main hall, with gold bricks on the ground, warm rooms at both ends, and a study room on the south. In history, the young Emperor Kangxi cleverly captured Oboi in the South Study Room of the Qianqing Palace, thus eliminating a serious threat and taking the power to govern the country into his own hands.