He honored his mother Lady Liu as princess dowager, and he immediately sought out an official Jin commission as the Duke of Xiping, and released the Jin messenger Yu Gui (俞歸), who had been detained by Zhang Chonghua in 347 after offending him by refusing to grant him the Prince of Liang title that he wanted.
Zhang Tianxi's reign was regarded as one that was filled by arbitrariness, as he apparently allowed Liu Su, Zhao Baiju, and other trusted followers, none of whom was older than 20 years at the time of his ascension, govern as they wished.
The experienced generals were largely offended at this development, and they did not dare to openly defy him but did not have sense of loyalty to him.
Later that year, Li Yan (李儼), a warlord who had occupied Longxi Commandery (隴西, roughly modern Dingxi, Gansu) and become a Former Qin vassal but who had also maintained contact with Former Liang, formally declared independence and cut off relations with Former Qin and Former Liang, occupying the commanderies around him.
Later that year, however, fearful that Former Qin had based large number of troops at his borders, he built an altar west of the capital Guzang (姑臧, in modern Wuwei, Gansu) and had his subordinates pledge an oath both to himself and to Jin, and then tried to petition the paramount Jin general Huan Wen to jointly attack Former Qin with him.
He sent a major force of 130,000 men, commanded by the general Gou Chang (苟萇) to head toward Zhang Tianxi's domain, but ahead of the force sent Yan and Liang again to try to persuade Zhang Tianxi to completely submit and visit the Former Qin capital Chang'an for an official visit.
Zhang Tianxi, believing that he would never be released if he went to Chang'an, decided to resist, and he cruelly executed Yan and Liang by ordering his officials to fire arrows at them, declaring, "If you cannot hit them, then you are showing you are not of the same heart as mine."
The other armies that Zhang Tianxi sent were all defeated by Former Liang forces, and the last one commanded by Chang Ju (常據) was annihilated.
Fu Jiān spared Zhang Tianxi and gave him a mid-level government post, creating him the Marquess of Guiyi.
Emperor Xiaowu of Jin made him a mid-level official and restored him to the title of Duke of Xiping, remembering his ancestors' loyalty.
He became known for his literary skills, but the Jin officials largely disrespected him for losing his state and being captured by Former Qin.
At some point, his mental state began to deteriorate (perhaps under Alzheimer's disease or some other illness that afflicted the mind) and was no longer given important posts.
Later, after Huan Xuan overthrew Sima Yuanxian, he wanted to use Zhang's family reputation for military purposes, and he gave Zhang the title of governor of Liang Province (涼州, modern central and western Gansu, Former Liang's domain, but which Jin did not control at the time).