He was most famous for his involvement in the crucial Battle of Fei River in 383, in which he set up a ruse that caused the disastrous retreat of the Former Qin army.
The Grand Marshal of Jin, Huan Wen appointed Zhu Xu to be Commander-Protector of the Expeditionary Force tasked in saving Chengdu.
The court issued Zhu Xu to campaign against the rebels after appointing him Marshal of the Centre Army and Administrator of Wuxing.
As Qin forces approached, Zhu Xu's mother, Lady Han (韓氏), personally inspected the Xiangyang's defence and noticed that the northwest corner was weak.
Without Zhu's knowledge, the Protector of Xiangyang, Li Bohu (李伯護), made contact with Qin, promising them to provide them an entry to the city.
[6] After being sent to the capital, Fu Jian was impressed by Zhu Xu's loyalty and appointed him Logistical Director of the Masters of Writing.
Fu Jian led a massive army to face the Jin forces at Fei River, which were commanded by Xie Xuan.
[9] As the battle was about to begin, Xie Xuan told Fu Jian to move his army slightly back away from the river to allow the Jin troops to cross.
Many of the Qin soldiers were conscripts who were not willing to fight, so upon hearing Zhu Xu's order, they began to panic and flee from the battlefield disorderly.
[10] After his return to Jin, Zhu Xu continued his service with the state, being first appointed as Prancing Dragon General and Interior Minister of Langye.
In 386, a Dingling chieftain, Zhai Liao, rebelled against Jin in Liyang (黎陽; northeast of present-day Xun County, Henan) after killing its administrator, Teng Tianzhi (滕恬之).
Zhu Xu ordered Qin Ying (秦膺) and Tong Bin (童斌) to lead troops against Zhai Liao.
He was then stationed in Pengcheng to replace Xie Xuan, although he later moved his base to Huaiyin, as it was closer to the capital and further from the borders of the powerful Later Yan.
Zhu Xu sent another request to the court, this time asking for supplies from 1,000 hectares of land from the office of the former Inspector of Jingzhou, Huan Shisheng (桓石生) as well as 80,000 hu of rice.
Zhu then sent Zhao Mu and Huan Bucai (桓不才) to pursue the fleeing Yan army, allowing them to defeat Yong at Taihang.
Murong Yong retreated to Shangdang, and not long after, Zhu Xu received the submission from the minor warlord named Yang Kai.
Zhu Xu continued to pursue Murong Yong as far as the White River (白水) in Shangdang, where they fought for 20 days.
The Jin administrators, Huangfu Zhao (皇甫釗) and Zhou Xun (周勳), surrendered three counties of Baxi (巴西; around present-day Langzhong, Sichuan) to Dou.
Jin's Inspector of Liangzhou, Zhou Qiong (周瓊), had little troops to resist them, so he asked Zhu Xu to send reinforcements.
[14] Near the end of his life, Zhu Xu requested the court for permission to resign multiple times, citing his old age and illness.