刘完素生活于金代，钻研《黄帝内经》「运气学说」多年，提出「六气皆能化火」「五志过极皆能生火」等理论，主张外感热病多从火热立论，善用寒凉药物清热泻火，形成「寒凉派」，是「河间学派」创始人，后世将他与张从正、李杲、朱震亨并称「金元四大家」。相传其学说经罗知悌辗转传至朱震亨，成为丹溪学派的重要理论渊源之一（这一传承环节部分史料记载不一，仅作参考）。

Living during the Jin dynasty, Liu Wansu spent years studying the theory of qi-transformation in the Yellow Emperor's Inner Canon and proposed that the six climatic factors could all transform into fire, and that extremes of emotion could likewise generate internal heat. He argued that externally-contracted febrile diseases were predominantly rooted in fire-heat and favored cooling, heat-clearing medicinals — founding the "Cooling School" and the Hejian lineage. He is grouped with Zhang Congzheng, Li Gao, and Zhu Zhenheng as the Four Great Masters of the Jin-Yuan period. His theories are traditionally said to have reached Zhu Zhenheng through an intermediate transmission via Luo Zhiti, though the exact links in that chain are recorded inconsistently across sources.