Little is known about Wamosi's early years, and it is not known when he was born. The first reference to him in Chinese historical sources, chronologically, was in 840, during the reign of Tang Dynasty's Emperor Wuzong. That year, Xiajiasi (Kirghiz) forces, under the Xiajiasi khan Are (阿熱), defeated and killed Huigu's khan Yaoluoge Hesa (藥羅葛闔馺) and chancellor Jueluowu (掘羅勿). The Huigu people scattered; some fled to the Geluolu (葛邏祿) tribes; some fled to Tufan; and some fled to Anxi (安西, in modern Aksu Prefecture, Xinjiang). Some of the Huigu people, led by Wamosi — who was said to be a brother of a khan (and the modern historian Bo Yang believed that he was Yaoluoge Hesa's brother), along with the chancellors Chixin (赤心), Pugu (僕固), and the noble Najiachuo (那頡啜), arrived at the Tang border city of Tiande (天德軍, in modern Bayan Nur, Inner Mongolia). They traded for food with other non-Han tribesmen, while seeking protection from Tang.
In spring 841, another group of Huigu remnants had supported another noble, Yaoluoge Wuxi (藥羅葛烏希), to be the new khan (as Wujie Khan). The defender of Tiande, Tian Mou (田牟) and eunuch monitor Wei Zhongping (韋仲平), wanting to crush Wamosi's group of Huigu in order to claim it as their achievement, thus claimed that Wamosi was a Huigu rebel and, based on the past alliance between Tang and Huigu, should be attacked. Most imperial officials agreed, but the lead chancellor, Li Deyu, pointing out that Wamosi had fled to Tang borders long before Wujie Khan claimed the khan title, argued that Wamosi was not a rebel. He advocated accepting Wamosi's submission. Emperor Wuzong, while not immediately doing so, ordered Tian not to provoke the Huigu, while ordering the armies of Hedong (河東, headquartered in modern Taiyuan, Shanxi) and Zhenwu (振武, headquartered in modern Hohhot, Inner Mongolia, which Tiande was part of) Circuits to mobilize to prepare to respond if the Huigu army attacked.
Meanwhile, in spring 842, Wamosi believed that Chixin would not be obedient to him, and therefore falsely informed Tian that Chixin was planning to attack Tiande. Tian responded by luring Chixin and Pugu into a trap and killing them. Najiachuo took some of the Huigu remnants and fled east. In the aftermaths, with various Huigu remnant groups, including the group under Wujie Khan, pillaging the northern Tang regions, Li Deyu advocated accepting Wamosi's submission, in order to encourage other Huigu nobles to submit. As a result, in summer 842, Wamosi was allowed to submit along with 2,200 other nobles. Wamosi was given a general title and was created the Prince of Huaihua.
Wujie Khan was continuing to pillage the Tang border regions, and was also demanding that Tang surrender Wamosi to him, a demand that Emperor Wuzong rejected. Meanwhile, Wamosi went to the Tang capital Chang'an to pay homage to Emperor Wuzong. Thereafter, Emperor Wuzong gave his army the name of Guiyi Army (歸義軍, i.e., "the army that submitted to righteousness") and made him the commander of the Guiyi Army. Apparently to further assure Emperor Wuzong of his faithfulness, Wamosi requested that his family members be kept at Hedong's capital Taiyuan Municipality and that he and his brothers be posted to the borders to help defend Tang. Emperor Wuzong ordered that the military governor (Jiedushi) of Hedong, Liu Mian (劉沔), to treat Wamosi's family with kindness. He also bestowed the Tang imperial clan name of Li on Wamosi and changed his name to Li Sizhong. (Wamosi's brothers Alizhi (阿歷支), Xiwuchuo (習勿啜), and Wuluosi (烏羅思) were given the names of Li Sizhen (李思貞), Li Siyi (李思義), and Li Sili (李思禮), respectively.)
In fall 842, with Wujie Khan and other Huigu remnants still posing threats, Emperor Wuzong ordered Liu, Li Sizhong, and Zhang Zhongwu the military governor of Lulong Circuit (盧龍, headquartered in modern Beijing), to rendezvous at Taiyuan to prepare for further operations. Li Sizhong subsequently volunteered to fight the Huigu remnants along with soldiers from the Qibi (契苾), Shatuo, and Tuyuhun tribesmen; in response, Emperor Wuzong ordered two prefects, He Qingchao (何清朝) and Qibi Tong (契苾通), to report to him. When, subsequently, in winter 842, Liu and Zhang requested a delay in the operations, but Li Zhongshun (李忠順) the military governor of Zhenwu requested that Li Sizhong attack the Huigu with him, Emperor Wuzong sent Li Sizhong to prepare for such an operation. (It is unclear, however, whether such an operation was actually ever launched.)
In spring 843, Li Sizhong went to Chang'an to again pay homage to Emperor Wuzong. Believing that the Tang border generals were suspicious of him, he requested that he, his brothers, as well as his ally, the noble Ai Hongshun (愛弘順), all be transferred to Chang'an. Emperor Wuzong agreed. Subsequently, the Guiyi Army was disbanded, with the Huigu soldiers being dispersed to various circuits. That was the last reference to Li Sizhong in Chinese historical records, and it is not known when he died.