After Gao Zhan became emperor in 561 (as Emperor Wucheng), He Shikai began to be promoted in earnest. When his mother Lady Liu died, Emperor Wencheng wept and sent the general Lü Fen (呂芬) to mourn Lady Liu on his behalf. He also soon summoned He Shikai back to the palace and ordered him and his four younger brothers to resume their official posts rather than observe the three-year mourning period for parents.
Emperor Wucheng suffered from a form of asthma that was particularly aggravated by drinking—and yet still drank. He Shikai often urged him to stop drinking, and on one occasion, when Emperor Wucheng was having breathing problems but nevertheless was about to drink, He Shikai wept heavily and was unable to speak. Emperor Wucheng responded, "You are making a speechless correction of me." He then stopped drinking. He Shikai had become so favored and trusted by Emperor Wucheng that Emperor Wucheng could not bear not seeing him, often requiring him to stay at the palace. Whenever He Shikai would go home, Emperor Wucheng would soon summon him to the palace again, and he rewarded He Shikai with great wealth. They participated in what were described as "immoral games" together, lacking boundaries between emperor and subject. He Shikai later started an affair with Emperor Wucheng's wife Empress Hu. However, one occasion at which Emperor Wucheng did not listen to He Shikai was in summer 562, when Emperor Wucheng's mother Empress Dowager Lou Zhaojun died, and he refused to wear the white mourning clothes and continued to feast and play music. When He Shikai requested that the music be stopped, Emperor Wucheng was displeased and slapped He Shikai.
He Shikai once stated to Emperor Wucheng:
- Ever since ancient times, all kings and emperors have turned to dust. What difference is there between Emperors Yao and Shun [two mythical kind emperors] and Jie of Xia and King Zhou of Shang [two legendary cruel kings]? your Imperial Majesty should, while you are still young and strong, enjoy life as much as possible and do whatever you wish. One day of extraordinary happiness is as great as one thousand years of ordinary living. Entrust the affairs of the state to the high-level officials, and do not worry that they would not be done. Do not mistreat yourself and make yourself unable to do anything.
Emperor Wucheng, persuaded, entrusted the civil service system to Zhao Yanshen (趙彥深), financial matters to Gao Wenyao (高文遙), the civilian administration over military affairs to Tang Yong (唐邕), and the education of Crown Prince Wei to Empress Hu's brother-in-law Feng Zicong (馮子琮) and cousin Hu Changcan (胡長璨). He himself only attended meetings with his ministers every three to four days, and he would often make short appearances and make several quick approvals and then end the meeting. Traditional historians usually viewed this as a major turnpoint leading to the degrading quality of Northern Qi imperial governance. Meanwhile, He Shikai became exceedingly powerful, and was known and praised for often finding ways to save other officials who were accused of crimes—but criticized for then extracting gifts from them. Further, because of He Shikai's tendency to overlook officials' offenses, the level of corruption in the Northern Qi government grew substantially.
Around 565, the official Zu Ting persuaded He Shikai that his fortunes were tied to the emperor's—and that if the emperor shall die, he would be in a desperate situation—and that he could solve this by suggesting Emperor Wucheng to pass the throne to his son, Gao Wei the Crown Prince, so that both the crown prince and Empress Hu would be grateful to him as well. He Shikai agreed, and both he and Zu offered the suggestion to Emperor Wucheng—stating to him that astrological signs indicating that the imperial position would be changed was a sign that he should pass the throne, particularly because it would be even more honored to be the father of an emperor than to be an emperor. Emperor Wucheng agreed, and in summer 565, he passed the throne to the eight-year-old Crown Prince Wei. Emperor Wucheng took the title Taishang Huang (retired emperor). (Zu subsequently had a falling out with He Shikai in 567, as he tried to have He Shikai, Zhao Yanshen, and Gao Wenyao removed, but in doing so offended Emperor Wucheng, and was imprisoned in a dungeon, where he lost his eyesight due to smoke from the Chinese cabbage seeds used for lighting.)
In spring 568, Emperor Wucheng suffered a major illness, and the official Xu Zhicai (徐之才), who was an accomplished physician, treated him back to health. After Emperor Wucheng recovered, however, He Shikai, whose position was lower than Xu's, wanted to be promoted, and so had Xu sent out to Yan Province (兗州, roughly modern Jining, Shandong) to be governor. In winter 568, Emperor Wucheng suddenly fell ill again, and he summoned Xu. Before Xu could arrive, however, around the new year 569, he died, while holding He Shikai's hands and entrusting the important matters to him.