Sun Xin, nicknamed “Little Yuchi,” belongs to the tightly bound Dengzhou family group in Water Margin. He keeps an inn outside the city with his formidable wife Gu Dasao and is the younger brother of the military instructor Sun Li. His decisive moment comes when the hunters Xie Zhen and Xie Bao are framed, imprisoned, and threatened with death. Gu Dasao insists on a rescue; Sun Xin backs her plan and helps bring relatives, innkeepers, and soldiers into a single conspiracy.
The most difficult recruit is Sun Li, whose official position makes open rebellion dangerous. Sun Xin and Gu Dasao lure him to the inn, stage a fierce confrontation, and force him to choose family loyalty over office. The group breaks the Xie brothers out of prison and flees Dengzhou. Soon afterward Sun Li pretends to defect to Zhu Family Village, while Sun Xin and the others enter with him as supposed dependants. When the signal comes, they attack from within and help Liangshan destroy a stronghold that had resisted two earlier assaults.
Sun Xin is therefore not presented as the equal of Sun Li in mounted combat. His weight lies in nerve, kinship, and the ability to turn an inn and a household into cover for dangerous action. At Liangshan he ranks one hundredth among the 108 Stars and holds an infantry command, often paired with Gu Dasao. He follows the army after amnesty and survives the long campaigns. Unlike many companions, husband and wife return to Dengzhou alive and receive appointments, giving the Dengzhou rescue story an unusually complete ending.
